Isothermal vapour-liquid equilibrium data (VLE) were measured for propionic acid + caproic acid, isobutyric acid + caproic acid, valeric acid + caproic acid, and caproic acid + enanthoic acid at several temperatures. A refined glass recirculating still with a packed equilibrium chamber and a vacuum-insulated, centrally located Cottrell pump was used for the measurements. Using the chemical theory for the vapour phase, true species concentrations (z(i)) were calculated for the monomers and dimers and from these liquid-phase activity coefficients by the methods of Prausnitz et al. [J.M. Prausnitz, T.F. Anderson, E.A. Grens, C.A. Eckert, R. Hsieh, J.P. O'Connell, Computer Calculations for Multicomponent Vapor-Liquid and Liquid-Liquid Equilibrium, Prentice-Hall, New Jersey, 1980], based on corresponding states theory and the Lewis fugacity rule for species fugacities. Rigorous tests showed the thermodynamic consistencies to be unusually high for all data sets. All systems showed positive deviations from Raoult's law and were satisfactorily modelled with the NRTL equation. (C) 2014 Elsevier B.V. All rights reserved.