We report the results of a series of hydraulic conductivity tests carried out on seven natural, well-characterized specimens of London Clay mudstone. The clay fractions of the samples range from 27% to 66% and enabled a test of the influence of clay fraction on the hydraulic conductivity, pore size distribution, compressibility and specific surface area of natural mudstones. Hydraulic conductivities were determined at effective stresses between 1.5 and 33 MPa. Hydraulic conductivities of clay-rich samples (49--66% clay fraction) decreased from ~10-11 m s-1 to ~10-14 m s-1 over a porosity range of 48% to 25%. At a given porosity the hydraulic conductivities of two silt-rich samples (27 and 33% clay fraction) were 40--250 times greater than those of the five clay-rich samples. Variations in hydraulic conductivity are directly related to pore size distributions and are accurately predicted by a model which uses pore size distribution as its primary input. Clay-rich samples have unimodal pore size distributions with modal throat radii around 60--120 nm. Silt-rich samples have bimodal pore throat size distributions. One modal size is similar to that observed in clay-rich samples with a second modal value at 3--6μm. Compaction under effective stresses up to 10 MPa results in the preferential collapse of larger pores, so that the rate of loss of hydraulic conductivity is greater in the silt-rich samples. Differences in hydraulic conductivity between silt-rich and clay-rich mudstones therefore decline with decreasing porosity. The range of porosity-hydraulic conductivity relationships means that hydraulic conductivity is not easily predicted from porosity alone; additional constraining parameters such as grain and pore size distributions are required. ¿ 1999 American Geophysical Union