Sequence placement of post-transcriptionally modified nucleosides in tRNA can be experimentally difficult, particularly in cases involving new or unexpected modifications or sequence sites. We describe a mass spectrometry-based approach to this problem, involving the following steps: crude isolations of one or several tRNAs by HPLC from an unfractionated tRNA mixture; digestion to oligonucleotide mixtures by RNase T1; analysis by combined HPLC/electrospray ionization-MS for recognition of modifications; and direct gas-phase sequencing of selected targets in the mixture by LC/MS/MS. Isoacceptor identity can be established in favorable cases when tRNA gene sequences are available.