The present work investigates the effect of polyol structure and physical addition of boric acid (BA) and N,N'-bis(2-hydroxyethyl)oxamide (BHEOD) on the properties of rigid polyurethane foams. The product of hydroxyalkylation of oxamide by ethylene carbonate (EC) has been used as a polyol component. The new polyol has been foamed using polymeric 4,4'-diphenylmethane diisocyanate (MDI), water and triethylamine (TEA). In order to decrease the flammability of the foams, BA and BHEOD were employed as the additive flame retardants. It has been found, that chemical modification of the foam structure by means of oxamide groups decreases their flammability only to a small extent, whereas physical addition of BHEOD does not influence the flammability. However, the addition of BA to the foam composition resulted in a distinct decrease of foam flammability, according to the amount of BA added. All the foams, modified and non-modified by boron, have been categorized into flammability class HF-1, according to the applicable standard.The introduction of flame retardants had its impact on the properties of polyurethane foams obtained, as described in this work.