The present invention relates to a phosphorus containing polyol, obtainable or obtained by a process comprising the reaction of at least one polyol with a phosphorus containing compound of the general formula (I) as defined herein, as well as the process for preparing a phosphorus containing polyol, comprising the reaction of at least one polyol with a phosphorus containing compound of the general formula (I). Furthermore, the present invention relates to the use of a phosphorus containing polyol as disclosed herein as a flame retardant, to a process for the preparation of a polyurethane and the polyurethane as such.