This paper presents data on the phase behavior of long-chain acids (octanoic acid, decanoic acid, dodecanoic acid, hexadecanoic acid, octadecanoic acid, and docosanoic acid) in supercritical propane. The data were measured for acid mass fractions W(acid) = (0.0149 to 0.652) in the temperature range T = (375 to 412) K, and the phase transition occurred at pressures in the range p = (3.7 to 11.2) MPa. The data showed no temperature inversions yet does hint toward possible three-phase regions at low temperatures for acids with 10 or more carbon atoms. The data shows a linear relationship between the phase transition pressure and the temperature at constant composition. Additionally, for acids with 10 or more carbon atoms, at constant temperature and mass fraction, a linear relationship exists between the phase transition pressure and the number of carbon atoms.