For a series of Diphenylmethane-4-4’-diisocyanate (MDI) and different polyols based polyurethanes (PU), oxygen index experiments (OI) and thermogravimetric analysis (TGA) were carried out. Linear correlations between some thermal parameters, e.g. initial decomposition temperature (IDT), mass loss at 500 degrees C and DTG-Max-Temperature and oxygen index (or oxygen concentration), were found. The most commonly used correlations for evaluation of the char residue were examined and a new coefficient is proposed. In a characteristic temperature range a pyrolysis was conducted; some volatile gas products were analysed.