A bis(pentaerythritol phosphate alcohol) alkylphosphonate compound, such as one comprising an alkyl group of from about one to four carbon atoms, preferably bis(pentaerythritol phosphate alcohol) methylphosphonate, is a flame retardant compound. Such a compound can be prepared by the transesterification of a diphenyl alkyl phosphonate carrying the desired alkyl group with pentaerythritol phosphate alcohol. An alternative process of preparation comprises the reaction of pentaerythritol phosphate alcohol, a trialkylamine, and an alkylphosphonic dihalide.