In this paper a conservative tracer (tritiated water) is used to assess the diffusive properties of 14 different laboratory-prepared soil–cement mixtures. A single-reservoir diffusion test setup to determine the effective porosity (ne) and the effective diffusion coefficient (De) through saturated, monolithic, soil–cement materials is used to assess the importance of mixture design on ne and De. Values of De and ne were found to range from 2.5 × 10−10 to 7.0 × 10−10 m2/s and from 0.21 to 0.41, respectively. Results indicate that the water content of the initial mixture has a substantial effect on the diffusive properties of the soil–cement material. The results suggest, from a contaminant transport perspective, that designers should generally aim to minimize the water content of cement-based solidification–stabilization materials.