For extrusive basalts of the upper oceanic crust, near-infrared radiation is reflected with a spectrum that is sensitive to the extent of crustal alteration. A published pilot study of basalts from Hole 801C showed that reflectance-based apparent alteration correlated with independent geophysical indicators of alteration (e.g., matrix density). This study shows that reflectance-based hydration correlates with geochemical measurements of structural water percentage and that air-dried archive half cores exhibit more reflectance-based hydration than do vacuumdried minicores. We calibrate the spectral response of visible and near-infrared spectroscopy (VNIS) with 907 measurements from seven basalt sites. The relationship between VNIS hydration estimates and geochemical measurements of structural water was determined, and a VNISbased estimate of relative smectite abundance was produced by factor analysis of three spectral characteristics. We used this calibration to estimate variations in hydration and smectite abundance throughout the 502-m section of upper oceanic crust at Ocean Drilling Program Site 1256, based on 2327 VNIS measurements on archive-half cores and 88 measurements on minicores.