AbstractBy dissociating noncovalent adducts with infrared radiation and minimizing ion charge density during measurement, Fourier‐transform mass spectrometry with electrospray ionization yields 160 000 resolving power for porcine albumin. Its molecular weight calculated from the published sequence is 66 740, but values found for two different samples are MW = 66 736 and 66 886. The spectrum of their mixture confirms this mass difference. Similar measurements of two Protein A (45 kDa) samples also give molecular weights in substantial disagreement with literature sequences. Tandem mass spectrometry gives partial sequence information that demonstrates errors in some of the reported sequence