The activity of Zn in low-melting {(1-x(1)-x(2))Sn + x(1)Zn + x(2)Ag} alloys has been determined by equilibrium saturation with metal vapour at T = 903 K. The total content of Zn and Ag was less than 0.3 mole fraction. The interaction parameter epsilon(Zn)(Ag) was determined by the least-squares method. As the method of equilibrium saturation with metal vapour is a comparative one, it is necessary to have an appropriate reference mixture. The {(1-x)Sn + xZn} alloy was assumed to be the appropriate one.