Description: Thermodynamic equations of stability and solubility of methane hydrate were developed using the method of Pitzer [Pitzer, K.S. (1991). Ionic interaction approach: Theory and data correlation. In: K.S. Pitzer (Editor), 2nd Edition, Activity Coefficients in Electrolyte Solutions. CRC Press, Roca Raton Ann Arbor Boston London, pp. 75–153.]. Dissociation pressures are calculated for different temperature and salinity conditions ranging from 273 to 293 K and 0–70 (salinity). The solubility of methane and methane hydrate in seawater is calculated for the same temperature and salinity ranges and for hydrostatic pressures (Pdis) up to 50 MPa. Since the composition of major pore water ions may change due to a variety of geochemical processes (i.e. anaerobic oxidation of organic matter or/and methane) affecting the activity of water, additional stability and solubility calculations are presented by substituting the equivalent amount of sulfate by hydrogen carbonate ions. Based on this rigorous thermodynamic analysis, the calculation of dissociation pressures has been approximated by empirical algorithms that are functions of temperature and salinity (chlorinity for pore water). Similar algorithms are presented for the calculation of methane concentrations in seawater and pore water equilibrated with methane hydrate as functions of salinity (chlorinity for pore water), temperature and hydrostatic pressure. In contrast to earlier approaches, the provided methods allow the calculation of these properties by easily applicable functions considering a continuous variation of the control parameters over a wide range of conditions that are met in the deep marine environment.