The effect of selected borates [i.e., tri(hydroxypropyl) borate (BTHP), tri[(3-chloro-2-hydroxy-lpropoxy)propyl] borate (BTC1HPP), and tri[(3-chloro-2-hydroxy-1-propoxy)-1-methylethyl] borate (BTC1HPME)] on the heat and thermal properties of the rigid polyurethane-polyisocyanurate foams was investigated. Increasing amounts of BTHP and BTC1HPP in the foam composition, from 0.1 to 0.4 of chemical equivalent, caused increases in the softening point, the temperature of the first decrement of foam mass, the extrapolated temperature of the main decrement of the foam mass, and temperature of the highest rate of the mass decrement. In the case when BTC1HPME was added to the foam compositions, the softening point decreased but temperatures characterizing their thermal resistance were higher compared to that of standard foam. (C) 2004 Wiley Periodicals, Inc.