The hydrocarbon composition of base oils produced by different processes - hydrotreatment (hydrocracking and wax isomerization), solvent refining plus hydrofinishing, and severe hydrofinishing - were determined by C-13 n.m.r. spectroscopy. Structural parameters such as normal and iso-paraffin contents, average chain length and number of branching sites were estimated by new equations derived after complete assignment of signals in the 5-21 ppm C-13 n.m.r. region. Quantitative differences in various types of branched structures among different categories of base oils were established. Relations between specific iso-paraffinic structures responsible for very high viscosity index and low pour point are discussed in terms of mobility behaviour and spin-lattice relaxation time measurements.