Hydrolysis of N3'-P5' phosphoramidate and thiophosphoramidate oligonucleotides with 0.1% formic acid leads to the cleavage of the 3' N-P bond and generates two products, one of which contains a 5'-phosphate. Analysis of the hydrolytic products by liquid chromatography, coupled with mass spectrometry, reveals the mass ladder from both termini, which is used to determine the sequence. While acid hydrolysis does not result in depurination, internal fragments especially in the low mass range are detected. The method is applied to DNA and RNA analogues with and without modifications at the 2'-position. This approach enables rapid sequence confirmation of synthetic phosphoramidate oligonucleotides for quality control as well as denovo sequencing.