A coupled one-dimensional model of the nitrogen and sulfur cycles in suboxic and anoxic conditions is developed and applied to the Black Sea redox zone and the oxygen-deficient layer of the Arabian Sea. The vertical distributions of nitrogen compounds (total organic nitrogen, ammonium, nitrate, nitrite) and inorganic reduced sulfur compounds (hydrogen sulfide, elemental sulfur, thiosulfate, sulfate) as well as dissolved oxygen agree reasonably well with the observations. The model takes into account the processes of turbulent diffusion, sedimentation, and biogeochemical transformation of compounds. Rates of biochemical processes mediated by bacteria are described by first-order equations using semiempirical functions of O2 concentration. The free nitrogen production in the model due to denitrification is estimated to be 34 Tg N yr-1 in the Arabian Sea and 1 Tg N yr-1 in the Black Sea; H2S oxidation and sulfate reduction in the Black Sea water column are in a balance of 1.6 mol m-2 yr-1.¿ 1997 American Geophysical Union