Interpretation of acoustic seismic records have allowed mapping of shallow gas accumulations and gas escape features in the Rias Baixas. X-ray photographs and voids of cores are semi-direct evidence of gassy sediments. Mapping of fluid-escape areas shows that these are related to the gas accumulations or at the intersections of faults. Analyses (GC-MS) of bubble samples collected in Simon Bay (Ria de Vigo) and across the whole ria confirm the presence of methane. The spatial distribution of gas escapes/accumulations and their vertical variations are interpreted as evidence of sedimentary facies control. The appearance of authigenic minerals (gypsum, pyrite and aragonite) and microbiological activity related to the seal facies are taken as evidence of the biogeochemical coupling processes. It is evident that these shallow-water coastal environments make significant contributions to the methane budget of the hydrosphere and atmosphere. It is also suggested that microbiological activity is favoured by gas escapes.