A composite oxidation catalyst (14, 16, 18, 20) 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 (44, 50) in the exhaust system, which composite oxidation catalyst comprising: a substrate, optionally 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 three or more catalyst washcoat zones (1, 2, 3; or 1, 2, 3, 4) arranged axially in series on and along the substrate surface, wherein a first catalyst washcoat zone having a length L 1, wherein L 1 < L, is defined at one end by the first substrate end and at a second end by a first end (19, 21) of a second catalyst washcoat zone having a length L 2, 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; the second catalyst washcoat zone comprises a second refractory metal oxide support material and one or more platinum group metal components supported thereon; and a third catalyst washcoat zone comprising a third refractory metal oxide support material and one or more platinum group metal components supported thereon is defined at a second end thereof by the second substrate end, 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, wherein a total platinum group metal loading in the third catalyst washcoat zone defined in grams of platinum group metal per litre of substrate volume (g/L) is less than the 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 supported on the first refractory metal oxide support material. a substrate, optionally 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 three or more catalyst washcoat zones (1, 2, 3; or 1, 2, 3, 4) arranged axially in series on and along the substrate surface, wherein a first catalyst washcoat zone having a length L 1, wherein L 1 < L, is defined at one end by the first substrate end and at a second end by a first end (19, 21) of a second catalyst washcoat zone having a length L 2, 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; the second catalyst washcoat zone comprises a second refractory metal oxide support material and one or more platinum group metal components supported thereon; and a third catalyst washcoat zone comprising a third refractory metal oxide support material and one or more platinum group metal components supported thereon is defined at a second end thereof by the second substrate end, 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, wherein a total platinum group metal loading in the third catalyst washcoat zone defined in grams of platinum group metal per litre of substrate volume (g/L) is less than the 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 supported on the first refractory metal oxide support material.