For modern biophysical methods such as isothermal titration calorimetry, high purity of the inhibitor of interest is indispensable. Herein, we describe a procedure for the synthesis and purification of functionalized phosphonamidate peptides that is able to generate inhibitors for the metalloprotease thermolysin for use in biophysical experiments. The method utilizes an allyl ester/alloc protection strategy and takes advantage of a fast and effective solid-phase extraction (SPE) purification step. Applying this strategy, we were able to synthesize a series of highly polar inhibitors featuring amino-and hydroxy-functionalized side chains in excellent purity.