The natural abundance 13C‐NMR spectra of a series of penicillins (penicillin V methyl ester, penicillin V, penicillin G, methicillin, oxacillin, cloxacillin, and dicloxacillin) were studied. The chemical shifts were assigned using the pulse Fourier transform technique with the aid of long‐range carbon‐13 hydrogen coupling. The previous assignments of penicillin V methyl ester were revised. Copyright © 1979 Wiley‐Liss, Inc., A Wiley Company