[reaction: see text]. Ammonium tert-butyl H-phosphonate was used for the phosphorylation of Tyr- and Ser-containing peptides synthesized by an Fmoc strategy. This reaction, leading to a monoprotected peptide phosphate, was found to be highly efficient and generally applicable. Moreover, the method employed avoids undesired side reactions during chain elongation (pyrophosphate formation and beta-elimination catalyzed by piperidine).