The ratio of the velocity of compressional waves, V p , to the velocity of shear waves, V s , is an important parameter for interpreting geophysical field data. Recent studies have emphasized the role played by pore geometry in controlling V p /V s in homogeneous rocks. We measured the carbonate content of a set of siliceous limestones of varying proportions of carbonate and silica and observed the pore structures of these samples using a scanning electron microscope. The range of V p /V s at 1.0 kbar is from 1.6 to 2.0. The behavior of V p /V s of individual samples during increasing confining pressure is consistent with crack-closure theory. However, the value of V p /V s within the sample set as a whole is dominated by its carbonate content. Variations in V p /V s due to total porosity and pore geometry are around 0.1, whereas the change due to composition is 0.4. Values of pore aspect ratios gained from comparison of the velocity-porosity-composition data with theory are in good agreement with the electron microscope observations.