In this paper, the total pressure is reported as a function of liquid-phase composition for n-pentane-methanol-2-butanol and each of its constituent binary systems at 303.15 K. The data were reduced by using Baker's method. The modified Margules equation was found to fit the binary measurements adequately. The ternary measurements were modeled with an expression for the excess Gibbs free energy which includes contributions from the binary systems plus additional terms containing ternary parameters.