A previously developed semi-empirical model for adiabatic two-phase annular flow (EPRI-NP-1521) is extended to predict critical heat flux in a vertical pipe for conditions of interest to boiling water reactors. The model exhibits a sharply declining curve of critical heat flux versus steam quality at low steam qualities where it is relatively independent of heat flux distribution. In this region, vaporization of the liquid film controls. At high steam qualities, net deposition upon the liquid film becomes important and the critical heat flux versus steam quality flattens considerably. In this zone, the critical heat flux is dependent upon heat flux distribution. Model predictions are compared to test data and an empirical correlation. The agreement is generally good if one employs previously reported mass transfer coefficients.