An hydroxytelechelic polybutadiene (PBHT) of 1200 molecular weight has been modified by the phosphonated thiol HS-(CH2)(3)-PO(OC2H5)(2) (I). The thiol (I) was prepared in three steps : phosphonation of allyl bromide, addition of thioacetic acid and hydrolysis. The grafting rate of (I) on PBHT has been calculated by sulfur analysis and by H-1 NMR. The results confirmed that the 1,2 double bonds are 10 times more reactive than the 1,4. The method led to new macromolecular polyols containing between 3 and 5 % in weight of phosphorous. These new long polyols should be very interesting in order to prepare fire retardant polyurethane networks.