The enthalpies of solute transfer from an HPLC mobile phase to the stationary phase were directly determined by flow microcalorimetry, i.e., without preparation of van't Hoff plots based on the temperature dependence of chromatographic retention data. Silica gel and hexane or binary mixtures of hexane and ethyl acetate were employed as the stationary phase and the mobile phases, and a homologous series of phthalates (dimethyl, diethyl, dipropyl, dibutyl, dipentyl, dioctyl and diphenyl phthalates) as the solutes. The flow system was contained with an isothermal microcalorimeter, set at 298 K, and included a reaction vessel (4 ml) into which a cylindrical cartridge (0.6 ml) was fixed. The phthalates dissolved in the solvent phases, equivalent to the mobile phases in the HPLC system, were flowed through the cartridge which contained silica gel and the generated power was recorded. With few exceptions negative values for the enthalpies of solute transfer were found; the values increased with increasing length of solute alkyl chain and with the content of ethyl acetate in the solvent phase. The enthalpies of solute transfer were also determined by a van't Hoff method and no discrepancy between the calorimetric and chromatographically based values was observed. (C) 2004 Elsevier B.V. All rights reserved.