Cretaceous rudists, having thick, multilayered bimineralic (aragonite and low-Mg calcite) shells, have been studied for the effects of phreatic/vadose meteoric diagenesis on their microstructural and isotopic attributes. Rudist skeletal components with preserved original mineralogy of aragonite and low-Mg calcite, and with preserved microstructures, have isotopic signatures analogous to Holocene marine bivalves. This similarity argues for a comparable isotopic composition of the Cretaceous and Holocene seawater, as well as for a similar mode of incorporation of stable isotopes into their shells. Diagenetic stabilization of rudist shells appears to have been a two-stage process, with low-Mg calcite --> diagenetic calcite (LMC --> dLMC), and particularly aragonite --> diagenetic calcite (A --> dLMC), transformation as the first step and cementation as a later development. The direct A --> dLMC mineralogical transformation (= calcitization) results in initial depletion in 18 O of <=2 per thousand and the subsequent diagenetic precipitation of ferroan calcite spar causes depletion in 13 C as well as an additional decrease in 18 O; the former due to introduction of organically derived CO 2 into the diagenetic aquifer. In contrast, 13 O and 13 C isotopic depletions in the originally LMC skeletal components are both related to the precipitation of the late diagenetic ferroan sparry calcite cement, the earlier calcitization stage being of subordinate importance only.