This paper describes a theoretical study of the self-condensation of non-equilibrium wet steam during supersonic expansion in a de Laval nozzle. Nucleation and droplet growth theories have been combined with the equations of one-dimensional gas dynamics and the system integrated numerically on a computer. The method of solution has been applied to predict the self-condensation zone in the supersonic region of a converging-diverging nozzle and it has been shown that the condensation of vapour in divergent part of the nozzle causes a pressure rise in flow. The results have been confirmed experimentally and, on the strength of this agreement, the treatment has been extended to predict the effects of friction factor, stagnation pressure and temperature on the location of the condensation shock. The work can be extended to predict the formation and growth of liquid droplets in steam turbines.