The vapour-liquid equilibrium behaviour of two carboxylic acid systems was studied; viz. propionic acid (1)-valeric acid (2) and isobutyric acid (1)-valeric acid (2). For each system, a single isobar at 20 kPa and three isotherms at 393.15, 403.15 and 413.15 K, respectively, were measured. Liquid-phase activity coefficients were determined using a procedure based on the chemical theory for the vapour phase similar to those of Tamir and Wisniak [A. Tamir, J. Wisniak, Chem. Eng. Sci. 30 (1975) 335-342], Marek [J. Marek, Coll. Czech. Chem. Commun. 20 (1955) 1490-1502], and Marek and Standart [J. Marek, G. Standart, Coll. Czech. Chem. Commun. 19 (1954) 1074-1084]. The experimental activity coefficients, gamma(A) and gamma(B), proved difficult to correlate using the commonly utilized Gibbs excess energy models, with best results obtained from the T-K Wilson equation. Area tests for thermodynamic consistency (strongly dependent on the chemical equilibrium constants, K) showed greater apparent consistency for the isobutyric acid-valeric acid system than for the propionic acid-valeric acid system. This is attributed to the possible formation of small amounts of propionic acid trimer. The vapour-phase correction factors, (Pi, showed considerable departure from unity, confirming the highly non-ideal nature of the vapour-phase for the mixtures studied. An efficient bubble-point calculation routine is proposed that yields system pressure and vapour-phase compositions (at any arbitrary liquid composition and temperature) of sufficient accuracy for distillation equipment design. (c) 2005 Elsevier B.V. All rights reserved.