Quasi-isochoric p rho T measurements on the vapor of acetic acid were performed in the low-density region by means of a quartz glass cell of constant volume. Nine isochores extending over the temperature range between 410 and 574 K were measured at densities from 20 up to 51 mol m(-3). Including data from literature experimental results are available in the temperature range between 298 and 574 K. Isothermal values needed in the evaluation were calculated in most cases from isochoric or isobaric data by means of a first-order Taylor series, in terms of temperature. The evaluation accounts for the fact that a strong dimerization depending on temperature occurs in acetic acid vapor. An exact statistic-mechanically based equation proposed by Lawson and Dahler as well as by Bich is used. The model considers the equilibrium and the density balance between monomers and dimers as constraints as well as the intermolecular forces between the components of the vapor. The temperature dependence of the dimerization constant and that of the enthalpy of dimerization are deduced for the temperature range between 298 and 500 K.