Solid samples of spent nuclear fuel were analyzed for actinide isotopic composition by resonance ionization mass spectrometry. Isotopes of U, Pu, and Am were simultaneously quantified using a new method that removes and/or resolves the isobaric interferences at 238U/238Pu and 241Pu/241Am without sample preparation other than cutting and mounting small (∼10 μm) samples. Trends in burnup and neutron capture product distributions were correlated with the sampling positions inside the reactor. The results show the skin effect, in which the core and near-edge regions of a fuel pellet exhibit strong differences in actinide concentrations and isotope distributions due to differences in the neutron energy spectra between the pellet rim and the core. While no elemental concentration measurements were made, the ability to measure the 238Pu/239Pu ratio in the presence of a 7400× excess of 238U enabled an estimate of the enhancement in Pu concentration due to the skin effect at the rim of the pellet.