Diagenesis of former aragonite and magnesian calcite marine cements of the Permian Reef Complex has resulted in a fine-scale intermixture of two distinct secondary phases, a luminescent and a nonluminescent calcite. Multiple carbon and oxygen isotopic analyses of these marine cements from any single sample result in a linear covariant trend, reflecting random proportions of the two calcite phases contained in individual analyses. The convergence of four trends derived from samples of former aragonite marine cement collected at four sites along a single Upper Capitan paleoslope fixes the isotopic composition of the nonluminescent calcite at a value of -2.5 per thousand delta 18 O, +5.3 per thousand delta 13 C. Likewise, covariant trends from former aragonite cements in the Lower Capitan and the youngest Upper Capitan yield nonluminescent calcite compositions of -2.8 per thousand delta 18 O, +5.2 per thousand delta 13 C and -0.7 per thousand delta 18 O, +5.8 per thousand delta 13 C, respectively. The single covariant trend generated from former magnesian calcite cements does not converge with equivalent former aragonite covariant trends, indicating that the precursor mineralogy influences the isotopic composition of the nonluminescent calcite. Considered as a whole, the invariance of the nonluminescent calcite signature, the timing of luminescent calcite emplacement, and the fine scale of the fabric retention suggest that the precipitation of the nonluminescent calcite occurred while isolated from effective isotopic exchange with external diagenetic fluids. A direct consequence of this low water/rock ratio, "closed" system is that the isotopic signature of the nonluminescent calcite also represents the original isotopic composition of the marine cement. This is further supported by the preservation of primary differences between the original isotopic compositions of aragonite and magnesian calcite cements. The constancy of the marine isotopic signature throughout the reef massive and foreslope facies requires that the Delaware basin water mass was relatively well mixed. Comparison of marine compositions throughout the range of the Capitan samples indicates that, although the oxygen composition of the marine cements was apparently constant through much of the Guadalupian, a substantial enrichment in delta 18 O occurred at the very end of the Guadalupian. This relatively abrupt change, recording a concomitant change in the oxygen composition of the Delaware Basin water mass, was probably the result of increased restriction of the Delaware Basin, and marked the first step in the sequence of events which led to the deposition of the deep-basin evaporites of the Castile formation.