13C NMR spectroscopy was used to investigate the structures of a series of pure isomeric cresylic novolak resins. Chemical shifts were measured and assigned, and end groups were distinguished from the central repeat units of the chains. Based on these measurements, the number average molecular weight of the para-cresylic oligomer was determined. A comparison of the 13C decoupled and coupled spectra of the three isomeric resins indicates that the para isomer has a linear, ordered structure in which the aromatic rings are linked only by ortho–ortho′ methylene-bridging groups. The structures of the ortho and meta isomers are more complex and contain ortho–ortho′, ortho–para′, and para–para′ methylene bridges.