Flame retardant oligomeric phosphate ester compositions having the structural formula: ##STR1## are prepared, wherein R is C.sub.1 -C.sub.10 alkyl or haloalkyl, R.sup.1 and R.sup.2 can be the same or different and are hydrogen, C.sub.1 -C.sub.10 alkyl or haloalkyl and n is an integer from zero to about 10.The method of preparing the compositions comprises reacting P.sub.2 O.sub.5 with a trialkyl or tris(haloalkyl)phosphate in the presence of about 0.01% to about 5% of a phosphite at a temperature from about 0.degree.-200.degree. C. A polyphosphate ester having P--O--P bonds is formed which is then reacted with an epoxide to yield the oligomeric phosphate ester flame retardant compositions.Alternatively, a method of preparing similar compositions comprises reacting P.sub.2 O.sub.5 with a trialkyl or tris(haloalkyl)phosphite at a temperature from about 0.degree. C. to about 200.degree. C. A mixed polyphosphate-polyphosphite ester having P--O--P bonds is formed which is then oxidized to yield a polyphosphate having P--O--P bonds; which is then reacted with an epoxide to yield the oligomeric phosphate ester flame retardant compositions.