A composite oxidation catalyst for use in an exhaust system for treating an exhaust gas produced by a vehicular compression ignition internal combustion engine, preferably for a heavy-duty diesel vehicle, and upstream of a particulate matter filter in the exhaust system, which composite oxidation catalyst comprising: a substrate, preferably a honeycomb flow-through substrate monolith, having a total length L and a longitudinal axis and having a substrate surface extending axially between a first substrate end and a second substrate end; and two catalyst washcoat zones arranged axially in series on and along the substrate surface, wherein a first catalyst washcoat zone having a length L 1 and comprising a first catalyst washcoat layer, wherein L 1 < L and L 1 is < 50% L, is defined at one end by the first substrate end and at a second end by a first end of a second catalyst washcoat zone having a length L 2 and comprising a second catalyst washcoat layer, wherein L 2 < L, wherein the first catalyst washcoat zone comprises a first refractory metal oxide support material and two or more platinum group metal components supported thereon comprising both platinum and palladium at a weight ratio of platinum to palladium of ≥1; and the second catalyst washcoat zone is defined at a second end thereof by the second substrate end and comprises a second refractory metal oxide support material and one or more platinum group metal components supported thereon, wherein the first and second catalyst washcoat layers abut one another without there being substantially any overlap between the first and second washcoat layers, wherein a total platinum group metal loading in the first catalyst washcoat zone defined in grams of platinum group metal per litre of substrate volume (g/L) is greater than a total platinum group metal loading in the second catalyst washcoat zone and wherein the first catalyst washcoat zone comprises one or more first alkaline earth metal components, preferably barium (Ba) or strontium (Sr), most preferably barium, supported on the first refractory metal oxide support material, wherein the composite oxidation catalyst comprises a washcoat overlayer extending axially for >50% up to 150% of the axial length of the underlying first catalyst washcoat layer from the first substrate end for protecting at least part of an underlying first catalyst washcoat zone from phosphorus and/or zinc poisoning when in use, which washcoat overlayer comprising a particulate metal oxide loading of >48.8 g/L (>0.8g/in 3) and wherein the particulate metal oxide in the washcoat overlayer has a mean pore diameter of ≥ 10 nm and/or the washcoat overlayer has a mean interparticle pore diameter of ≥ 10 nm, wherein the particulate metal oxide in the washcoat overlayer supports a platinum group metal, which is platinum or a combination of platinum and palladium at a Pt:Pd weight ratio of ≥ 1:1. a substrate, preferably a honeycomb flow-through substrate monolith, having a total length L and a longitudinal axis and having a substrate surface extending axially between a first substrate end and a second substrate end; and two catalyst washcoat zones arranged axially in series on and along the substrate surface, wherein a first catalyst washcoat zone having a length L 1 and comprising a first catalyst washcoat layer, wherein L 1 < L and L 1 is < 50% L, is defined at one end by the first substrate end and at a second end by a first end of a second catalyst washcoat zone having a length L 2 and comprising a second catalyst washcoat layer, wherein L 2 < L, wherein the first catalyst washcoat zone comprises a first refractory metal oxide support material and two or more platinum group metal components supported thereon comprising both platinum and palladium at a weight ratio of platinum to palladium of ≥1; and the second catalyst washcoat zone is defined at a second end thereof by the second substrate end and comprises a second refractory metal oxide support material and one or more platinum group metal components supported thereon, wherein the first and second catalyst washcoat layers abut one another without there being substantially any overlap between the first and second washcoat layers, wherein a total platinum group metal loading in the first catalyst washcoat zone defined in grams of platinum group metal per litre of substrate volume (g/L) is greater than a total platinum group metal loading in the second catalyst washcoat zone and wherein the first catalyst washcoat zone comprises one or more first alkaline earth metal components, preferably barium (Ba) or strontium (Sr), most preferably barium, supported on the first refractory metal oxide support material, wherein the composite oxidation catalyst comprises a washcoat overlayer extending axially for >50% up to 150% of the axial length of the underlying first catalyst washcoat layer from the first substrate end for protecting at least part of an underlying first catalyst washcoat zone from phosphorus and/or zinc poisoning when in use, which washcoat overlayer comprising a particulate metal oxide loading of >48.8 g/L (>0.8g/in 3) and wherein the particulate metal oxide in the washcoat overlayer has a mean pore diameter of ≥ 10 nm and/or the washcoat overlayer has a mean interparticle pore diameter of ≥ 10 nm, wherein the particulate metal oxide in the washcoat overlayer supports a platinum group metal, which is platinum or a combination of platinum and palladium at a Pt:Pd weight ratio of ≥ 1:1.