The effects of three selected borates {tri(hydroxypropyl), tri[(3-chloro-2-hydroxy-l-propoxy)-1-methylethyl], tri[(3-chloro-2-hydroxy-l-propoxy)propyl]} on the heat and thermal properties of rigid polyurethane-polyisocyanurate foams was studied. Increasing the amount of tri(hydroxypropyl) borate and tri[(3-chloro-2-hydroxy-lpropoxy)propyl] borate in the foam composition from 0.1 to 0.4 chemical equivalents caused an increase 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 the temperature of the highest rate of the mass decrement. When tri[(3-chloro-2-hydroxy-l-propoxy)1-methylethyl] borate was added to the foam compositions, the softening point decreased but the temperatures characterizing their thermal resistance were higher in comparison with the standard foam. (c) 2006 Wiley Periodicals, Inc.