The changes in porosity of OPC and an OPC-fly ash blended cement during hydration have been studied at water/solids ratios of 0.35, 0.47 and 0.59, cured for times of up to 1 year at 25°C. The porosity was measured indirectly by methanol exchange and methanol adsorption techniques and, directly, by quantitative image analysis using backscattered electron imaging in the scanning electron microscope. Measurements of porosity and of remaining anhydrous material by image analysis showed good correlation with indirect methods. Measurement of the diffusion of methanol and of the compressive strength were made in parallel with the determination of the porosity during hydration and attempts were made to relate the properties to the microstructure. For both binders the reduction of total porosity with increased reaction was small. The major change in pore structure was the subdivision of coarse pores by gel to form finer pores. Compressive strength and diffusion properties were dominated by the relative volume of coarse pores.