Vapor-phase association greatly affects the pure-component properties of carboxylic acids. This study uses density functional theory (DFT) calculations to quantify the size and distribution of oligomers present in n-aliphatic carboxylic acid vapors and includes recommendations on how this information may be used in modeling the acids. Acids from C2 to C6 were studied using B3LYP/6-31++G(2d,p). New association equations were developed for the acids from C4 to C10 using the family trend obtained from the B3LYP calculations. Dimers were found to be the dominant oligomer in the vapor-phase, with little association of larger oligomers. Association in acids larger than C10 is negligible at modest temperatures and pressures. There is a notable difference in the dimerization enthalpy between acids with an even number of carbon atoms and those with an odd number of carbon atoms. (C) 2004 Elsevier B.V. All rights reserved.