In differential ebulliometry the true liquid equilibrium composition is related to the charge composition through an equipment characterization parameter E, which is a function of the vaporization ratio phi. Exact computation of gamma (infinity)(1) requires an iterative solution of a set of three equations. A similar but simpler formulation is possible for differential static apparatus, which accounts exactly for the vapor hold-up. An equipment parameter E is most conveniently resolved in terms of the total equipment volume and involves two new dimensionless groups R-T and R-G. An iterative procedure involving regula-falsi gave rapid convergence for all systems studied. The flexible one-constant Wilson equation is recommended. For systems of large relative volatility large errors in gamma (infinity)(1) will result, even at low pressure, if vapor hold-up is not accounted for and if R-T much greater than 1. New values for the self-association constant k are proposed for 2-propanol and ethanol as functions of temperature.