Material balance calculations for gas-condensate reservoirs with reservoir pressures below the dew-point pressure require the use of two-phase deviation factor to account for the phase behaviour effects of a gas-condensate fluid. Recently, an empirical correlation for the two-phase deviation factor has also been presented in the literature. In this study, a phase behaviour package based on the Redlich-Kwong equation of state as modified by Zudkevitch and Joffe has been used to investigate some of the issues in connection with the application of two-phase deviation factor to material balance calculations for gas-condensate reservoirs. This study shows that the two-phase deviation factor does not necessarily increase in the two-phase region as the pressure increases, whereas the empirical correlation shows that the two-phase deviation factor always increases with pressure. A study has also been carried out to quantify the errors in estimating the initial gas-in-place due to input data errors, fluid composition effects, computation methodology for deviation factor, and the use of improper deviation factor.