Both the extent of SiO2 doping and the mesopore size of alumina were investigated with the aim of developing a support that maintains high activity during the catalytic oxidation of diesel exhaust hydrocarbons and NO over 0.75 wt % Pt/0.25 wt % Pd even after high temperature treatment. By controlling the SiO2 doping ratio and heat treatment conditions during the preparation process, 24 types of SiO2-Al2O3 were obtained, with SiO2 levels from 0 to 5.9 wt % and mesopore sizes from 6.7 to 10.2 nm. The highest catalytic activity was obtained at a SiO2 loading of approximately 4 wt %, and the beneficial effects of SiO2 doping were thought to result from optimization of the basicity of the support. Catalytic activity improved as the mesopore size increased up to approximately 10 nm. Computational simulations confirmed that the diffusion limitations of small mesopore structures may affect catalytic activity when the mesopores are below 10 nm.