Detailed lithological, biogeochemical and molecular biological analyses of core sediments collected in 2002-2006 from the vicinity of the Malenky mud volcano, Lake Baikal, reveal considerable spatial variations in pore water chemical composition, with total concentrations of dissolved salts varying from 0.1 to 1.8‰. Values of methane δ¹³C in the sediments suggest a biogenic origin (δ¹³C_min. -61.3‰, δ¹³C_max. -72.9‰). Rates of sulphate reduction varied from 0.001 to 0.7 nmol cm^-3 day^-1, of autotrophic methanogenesis from 0.01 to 2.98 nmol CH₄ cm^-3 day^-1, and of anaerobic oxidation of methane from 0 to 12.3 nmol cm^-3 day^-1. These results indicate that methanogenic processes dominate in gas hydrate-bearing sediments of Lake Baikal. Based on clone libraries of 16S rRNA genes amplified with Bacteria- and Archaea-specific primers, investigation of microbial diversity in gas hydrate-bearing sediments revealed bacterial 16S rRNA clones classified as Deltaproteobacteria, Gammaproteobacteria, Chloroflexi and OP11. Archaeal clone sequences are related to the Crenarchaeota and Euryarchaeota. Baikal sequences of Archaea form a distinct cluster occupying an intermediate position between the marine groups ANME-2 and ANME-3 of anaerobic methanotrophs.