Heat effects resulting from the introduction of solid human serum albumin (HSA) into various water-acetonitrile mixtures were measured calorimetrically at 298 K. The amount of water bound to the suspended HSA as a function of the water content of the solvent was also determined. Introducing HSA into water-acetonitrile mixtures involves water binding according to the Langmuir isotherm with an adsorption constant K-c = 1.0 +/- 0.1 M(-1), enthalpy Delta h = -9.0 +/- 1.5 kJ mol(-1) and entropy Delta S = -30 +/- 6 J mol(-1) K-1. Placing HSA in the solvent has an additional heat effect of 46 +/- 19 J g(-1), which is attributed to an unknown transformation of the protein preparation.