CH 4 and CO 2 species in pore fluids from slope sediments off Guatemala show extreme 13 C-enrichment (δ 13 C of -41 and +38‰, respectively) compared with the typical degree of 13 C-enrichment in pore fluids of DSDP sediments (δ 13 C of -60 and + lO‰). These unusual isotopic compositions are believed to result from microbial decomposition of organic matter, and possibly from additional isotopic fractionation associated with the formation of gas hydrates. In addition to the isotopic fractionation displayed by CH 4 and CO 2 , the pore water exhibits a systematic increase in δ 18 θ with decrease in chlorinity. As against seawater δ 18 θ values of 0 and chlorinity of 19‰, the water collected from decomposed gas hydrate from Hole 570 had a δ 18 θ of + 3.0‰ and chlorinity of 9.5‰. The isotopic compositions of pore-fluid constituents change gradually with depth in Hole 568 and discontinuously with depth in Hole 570.