Oxygen isotopic compositions were determined for coexisting mixed-layer serpentine-chlorite (Sp-Ch) and illite-smectite (I-S) from 5 Tuscaloosa Formation sandstone cores sampled between 1937 and 5470 m burial depth. High gradient magnetic separation (HGMS) was used to concentrate Sp-Ch and I-S from the <0.5 mu m fraction of each core sample into fractions with a range in the Sp-Ch:I-S ratio, and end-member delta 18 O compositions were determined by extrapolation. The Sp-Ch delta 18 O values range from +10.4 to 13.7 per mil and increase with burial between 3509 and 5470 m. The only exception is Sp-Ch from 1937 m, which has an anomalously high delta 18 O value of +12.6 per mil. The I-S delta 18 O values range from +16.1 to 17.3 per mil and do not change significantly between 3509 and 5470 m burial depth. Pore water delta 18 O compositions calculated from Sp-Ch and I-S values and measured borehole temperatures range from -2.6 to +10.3 per mil. The isotopically light values indicate that Sp-Ch formed at shallow burial depths in the presence of brackish to marine water and/or meteoric water. The depth-related increase in delta 18 O of Sp-Ch is attributed to oxygen exchange between mineral and pore water during diagenetic mineral reactions. Increasing delta 18 O values, in conjunction with XRD and SEM data, indicate that transformation of serpentine layers to chlorite layers and Ibb polytype layers to Iaa polytype layers occurred on a layer-by-layer basis when individual layers dissolved and recrystallized within the confines of coherent crystals. Possible explanations for the variation in I-S delta 18 O values include depth-related differences in pore water delta 18 O values present at the time of I-S crystallization, contamination by detrital 2M 1 mica and 1M polytype rotations that facilitated oxygen exchange.