A quantitative P-31-NMR method for the determination of fosfomycin and impurity A in pharmaceutical products of fosfomycin sodium or calcium has been developed. In this method, coaxial inserts containing trimethyl phosphate are used as external standard. The method is convenient and robust, and gives both high accuracy and precision. It is shown that an accurate determination is possible using different probes and coaxial inserts. Copyright (c) 2015 John Wiley & Sons, Ltd.