The vapor-phase compressibility factors of several monocarboxylic acids (formic acid, acetic acid, propionic acid, n-butyric acid, isobutyric acid, and valeric acid) were measured by a flow-type apparatus at saturated vapor pressure from 30 degrees C to 150 degrees C. The vapor-phase association constants for monocarboxylic acids were evaluated from the compressibility factors obtained. They can be expressed in In K = -149/R + 58 500/RT for formic acid and In K = -136/R + 58 500/RT for other acids, where K is the association constant, T is the temperature in K, and R is the gas constant in J.mol(-1).K-1.