The P-p-T-x relationships of the propane-n-octane and the n-butane-n-octane systems are determined. The experimental results cover a range from about 150 lb/in.2 abs and room temperature to the highest pressure and temperature at which liquid and vapor can coexist. The data are presented in tabular form. P-T-x, p-T-x, and isobanc T-x diagrams are given.This paper presents the P-p-T-x relationships of the binary systems of propane-n-octane and n-butane-n-octane. It is the third of a series (5, 6) in a study of the effect of the relative size of the molecules on the phase diagram of mixtures of n-alkanes.