The method of boroorganic compounds preparation by estrification of boric acid and various glycols has been described. The borates obtained i.e. tri(2-hydroxypropyl) borate - THPB, tri(2-hydroxybutyl) borate - THBB and tri(thiodiglycol) borate were applied to prepare the rigid polyurethane - polyisocyanurate foams (PIR-PUR). The foams were prepared by one-stage method and the amount of borates applied was various from 0.0 to 0.4 of the chemical equivalent. The borates used to prepare the foam composition caused the structure to become ordered. It favoured the reduction of their brittleness. Addition of borates reduced the flammability of foams what resulted in the increase of oxygen index. Introducing THPB and THBB into the foam composition caused a significant increase in compressive strength but addition of TTDGB caused its decrease. It is probably due to a long TTDGB chain containing sulphur atoms in its middle part what results in plastifization of foams. Our studies have shown tri(2-hydroxypropyl) and tri(2-hydroxybutyl) borates to be useful as an addition improving the polyurethene - polyisocyanurate foam properties; however, the usability of tri(thiodiglycol) borate is limited.