The relative timing of ore metal accumulation and clay formation in sandstone-hosted vanadium-uranium (V-U) deposits is poorly known. The relation between these two processes was investigated by comparison of the mineralogy, chemical composition, petrography, and Rb-Sr ages of clays in barren and ore-bearing sandstones from the Middle Jurassic Entrada Sandstone. The only clay detected in all samples was illite that is aluminous, a 1M polytype, and contains variable but low amounts of interlayered smectite (< 15%). Barren sandstone typically contains <7 percent illite that includes detrital clay and authigenic fibrous and ribbonlike overgrowths. Within the V-U layers, the only clay is authigenic, vanadiferous (> 1 wt % V 2 O 3 ) illite (roscoelite), which forms ribbons and plates that locally constitute as much as 70 percent of the rock. Rb-Sr data failed to produce a usable isochron presumably because of heterogeneity in the initial 87 SR/ 86 Sr. Calculated model ages for barren and vanadium-rich illites are similar and most values range from 30 to 45 Ma. An episode of illitization that altered preexisting V-U deposits produced the similar characteristics of illite in barren and ore samples. Geochemical constraints support accumulation of vanadium prior to the Late Cretaceous, when the Entrada Sandstone was buried by less than 300 m of overlying sediments. Organic compounds or HiS reduced chromium, uranium, and vanadium dissolved in ground water and precipitated them as oxides or precursor clay minerals. Later, during the Tertiary, illite formed by reaction between epigenetic solutions at approximately 100%C, and solid constituents of the Entrada Sandstone. The unusually large abundance of illite in ore relative to barren sandstone is consistent with recrystallization of precursor clay minerals or enhanced illite precipitation because of a high dissolved vanadium activity. Based on this interpretation, genetic models of V-U deposits should not assume that ore clays formed coeval with metal accumulation.