The circulation of cold water through the oceanic crust at mid-ocean ridge flanks causes changes in the composition and physical properties of the crust. This study focuses on the effect of hydrothermal ridge flank alteration on in situ physical properties. P wave velocity, electrical resistivity, and natural radioactivity are compared. They are obtained from downhole logging operations in seven boreholes drilled by the Deep Sea Drilling Project and Ocean Drilling Program. Basement age ranges from 5.9 to 167 m.y. The comparison shows that the average values of P wave velocity and electrical resistivity increase with the logarithm of basement age for lava flows and pillow basalts. Natural radioactivity and potassium content do not show a statistically significant relation to the basement age. No direct influence of spreading rate or thickness of the sediment cover on the in situ physical properties can be observed. The increase in P wave velocity with age reflects the reduction in pore space due to the precipitation of secondary minerals. Electrical resistivity is low in young crust where conductive clays are important alteration minerals. It increases in old crust because electrically insulating minerals, such as carbonates, are formed as late stage alteration minerals. Natural radioactivity is influenced by the composition of the magma but also rises in altered rocks due to the formation of K-rich clay minerals. The results of this study show that ridge flank hydrothermal alteration persists over longer-term intervals than indicated by seismic experiments.