Electrospray ionization coupled with Fourier-transform mass spectrometry routinely achieves sufficiently high mass accuracy (less than or equal to 20 ppm) and resolving power (10(5)) to provide base composition and sequence information from oligonucleotides as large as 25 bases. Nozzle skimmer (NS) dissociation yields fragment anions that describe, consistent with the McLuckey mechanism, the complete sequence of 8-14-base oligomers and extensive sequence information for a 21-mer and a 25-mer. The spectrum from collisionally activated dissociation of an 8-mer is closely similar to its NS spectrum. Several spectra per minute can be measured, predicting a key role for ESI/MS in nucleotide sequence verification.