DSDP Sites 501/504 and 505 are located in a continous segment of the southern flank of the Costa Rica Rift. Sediments at the two localities, although similar in composition, thickness and depositional environment sharply differ in the degree to which they have undergone diagenesis. At Sites 501/504 the unconsolidated ooze of the uppermost sediments turns to a firm chalky ooze below 144 m depth and then sharply to limestone and chert at 227 m with lithification further increasingly downwards. At Site 505 no diagenetic boundaries are present. The oxygen isotopic compositions of pore waters and carbonate sediments from the two localities are very different. The delta (super 18) O of the pore waters from Sites 501/504 are markedly depleted with respect to SMOW whereas those of Site 505 are very close to SMOW. The delta (super 18) O of carbonates from both sites are similar except for the deepest samples. At 230 m depth (Site 501) the delta (super 18) O of the carbonates drop to values as low as 27 per mil. It seems likely that this change is a reflection of carbonate recrystallization. At Site 501 the delta (super 18) O decrease in carbonates is matched by an increase in the delta (super 18) O of pore fluids in Sites 501/504. The delta (super 18) O of the pore fluids decrease to a depth of 230 m where a reversal occurs. The inverse match suggests the two delta (super 18) O changes are interrelated.