Possibility of theoretical calculations of Henry's constants by equations of state and mixing rules has been analysed for strong nonideal systems. Five cubic equations of state with quadratic and Huron-Vidal mixing rules were used for systems with water as the solvent. Values of Henry's constants calculated by Edwards et.al. correlation were assumed as the reference states. The calculations proved that method without cross interaction coefficients gives entirely incorrect results. Using one temperature independent cross coefficient considerably improves the results. Quite good agreement was obtained, by applying two temperature independent cross interaction coefficients. Method with Huron-Vidal mixing rules gives better results than method with van der Waals mixing rules.