Display omitted ► Model-free estimation of K factors at infinite dilution. ► Useful for interpolation of phase boundary curves. ► Simple consistency test for isothermal phase diagrams. ► Special behaviour of phase boundary curves for limiting azeotropy. The extended van’t Hoff equations for dilute mixtures relate the differences of initial slopes of phase boundary curves to the phase transition enthalpy or volume of the solvent. While the isobaric van’t Hoff equation is frequently used in mixture thermodynamics, its isothermal counterpart has received much less attention. We show that the isothermal van’t Hoff equation can be used to interpolate dew point curves in isothermal phase diagrams or to create a model-free consistency test for experimental phase equilibrium data. Furthermore, an expression for the initial curvature of phase boundary curves is derived.