Precise isothermal vapor-liquid equilibrium data at 10, 30, 55 and 70 degrees C for the system water + tert.-pentanol were measured using a computer-operated differential static apparatus, Activity coefficients at infinite dilution were derived from the experimental P - x data in the dilute region using a flexible Legendre polynomial, and the vapor-liquid-liquid locus was derived directly from the P - x data near the liquid-liquid phase boundary, Heteroazeotropic points were measured directly by distillation using a rotating band column, Furthermore the UNIQUAC and the NRTL models were used to correlate the experimental P - x data and to derive the azeotropic data. Experimental H-E data were taken from literature and used together with the experimental P - x data to simultaneously fit temperature dependent interaction parameters for UNIQUAC and NRTL. The parameters were used to predict the azeotropic composition over a large temperature range, The results were compared with those of a simple analytical thermodynamic equation using only the pure component vapor pressure data, heats of mixing in the heterogeneous region and the azeotropic composition at one temperature. Heats of mixing were measured at 140 degrees C with the help of a flow calorimeter in order to determine the slope of H-E VS. x(1) in the heterogeneous region, The H-E data were used to check the reliability of the G(E) model parameters and the equation to calculate the temperature dependence of the heteroazeotropic composition.