A process for making a phosphorus-containing compound is disclosed. The process comprises contacting a compound of formula (A) wherein RA and RB are selected from optionally substituted aryl, aryloxy, alkyl and alkoxy groups or can be combined to form cyclic structures; and RC is methyl, ethyl, isopropyl, n-butyl, i-butyl, t-butyl, phenyl or benzyl; and a compound of formula (B) wherein R1-R4 are selected from optionally substituted aryl, aryloxy, alkyl and alkoxy groups. The phosphorus-containing compound can then be used as a flame retardants for polymers, especially for epoxy, polyurethane, thermosetting resins and thermoplastic polymers. Such flame retardant-containing polymers can be used to make protective coating formulations and ignition-resistant fabricated articles, such as electrical laminates, polyurethane foams, and various molded and/or foamed thermoplastic products.