For the first time, the contents of isolated methyl groups, i.e. methyl groups bound to quaternary aliphatic or aromatic carbon, in crude oil samples were determined. As an analytical tool we used the (3)J(H,H) modulated H-1-NMR spin echo technique. Weighed amounts of hexamethyldisiloxane (HMDS) containing six isolated methyl groups were added to the samples to determine absolute concentrations. The contribution of isolated methyl carbon to total carbon was in the range of 3.0 to 6.9% methyl groups bound to aliphatic carbon, and 1.2 to 4.6% for methyl bound to aromatic carbon. Model compound mixtures of known composition were prepared to validate the new procedure. The procedure delivers accurate and precise results with regard to relative standard deviations of less than 1%. (C) 2002 Elsevier Science Ltd. All rights reserved.