The phase transition in poly(N-vinylpyrrolidone) (PVP) aqueous solutions is shown to occur at heating upon addition of organic acids such as isobutyric, isovaleric, and, especially, trichloroacetic (TCA) ones. The cloud point temperature (T-c) of PVP solutions drops from 70 to 6 degrees C when the TCA concentration rises from 0.2 to 0.3 mol/l. A decrease in T-c is even more drastic when HCl is also added though HCl addition to the system without TCA does not result in phase separation. These phenomena are explained by the reversible coordination between the non-ionized form of TCA and PVP units via hydrogen bonding. An increase in the medium acidity depresses TCA dissociation, resulting in an increase in PVP-TCA associate concentration. Calculations based on the pK(a) values of TCA confirm this suggestion. The similar behavior is observed with poly(N-vinylcaprolactam) systems. The amount of TCA bound to PVP has been determined by means of separation of the precipitate by centrifugation at temperatures above T-c and subsequent titration of TCA in the polymer with NaOH. It is shown that the precipitate contains one TCA molecule per 3-6 VP units, this value decreasing down to 1.25-2 upon HCl addition to the system. (C) 2008 Elsevier Ltd. All rights reserved.