Abstract— <p>This work presents new data on the composition and concentrations of hydrocarbon gases in the upper horizons of bottom sediments of Avacha Bay and the adjacent zone of the Pacific Ocean. The established concentrations of methane and the sums of its homologues are highly variable, from 0.0003 to 120.9385 and from 0.00005 to 0.00753 cm³/kg. Gas anomalies are formed in the sediments of the shallow shelf (Avacha Bay, Russkaya Bay, Vilyuchinskaya Bay and Listvennichnaya Bay) and in the outer shelf and the continental slope at sea depths of 14–800 m and sampling depths of 3–50 cm. The data on the isotopic composition of carbon δ¹³C–CH₄ from –78.1 to –33.4‰ and δ¹³C–CO₂ from –19.8 to –11.1‰ indicate the formation of natural gases of various polygenetic (mixed) origin in the bottom sediments of the region and, with a high probability, their significant influx from the underlying sediments.</p>