An apparatus for treating a flow of exhaust gas, including: at least one gas permeable body (10A; 10B), the at least one gas permeable body having at least one upstream surface, at least one downstream surface, and a total section area normal to the flow; a flowpath for directing the flow to enter the at least one gas permeable body (10A; 10B) via the at least one upstream surface and to flow through the at least one gas permeable body (10A; 10B) from the at least one upstream surface to the at least one downstream surface to heat the at least one gas permeable body (10A; 10B) to an operating temperature; and a flow diverter mechanism operable to selectively divert the flow between at least first and second regions (10A; 10B) of the at least one gas permeable body, the first and second regions (10A; 10B) defining respective, first and second portions of the total section area; characterised in that the flow diverter mechanism is arranged, when commencing treatment from a cold condition of the at least one gas permeable body: (i) to direct at least a major proportion of the flow through at least the first region but not the second region to heat the first region to the operating temperature; and then, after the first region has reached the operating temperature, (ii) to direct at least a major proportion of the flow through at least the second region but not the first region to heat the second region to the operating temperature; and then, after the second region has reached the operating temperature, (iii) to reduce the proportion of the flow through the second region and to direct a proportion of the flow through the first region so that the flow is shared between the first and second regions (10A, 10B). at least one gas permeable body (10A; 10B), the at least one gas permeable body having at least one upstream surface, at least one downstream surface, and a total section area normal to the flow; a flowpath for directing the flow to enter the at least one gas permeable body (10A; 10B) via the at least one upstream surface and to flow through the at least one gas permeable body (10A; 10B) from the at least one upstream surface to the at least one downstream surface to heat the at least one gas permeable body (10A; 10B) to an operating temperature; and a flow diverter mechanism operable to selectively divert the flow between at least first and second regions (10A; 10B) of the at least one gas permeable body, the first and second regions (10A; 10B) defining respective, first and second portions of the total section area; (i) to direct at least a major proportion of the flow through at least the first region but not the second region to heat the first region to the operating temperature; and then, after the first region has reached the operating temperature, (ii) to direct at least a major proportion of the flow through at least the second region but not the first region to heat the second region to the operating temperature; and then, after the second region has reached the operating temperature, (iii) to reduce the proportion of the flow through the second region and to direct a proportion of the flow through the first region so that the flow is shared between the first and second regions (10A, 10B).