This paper reviews analytical methods that have been developed for characterizing complex liquid mixtures derived from fossil fuels. The analysis of fractions with masses up to similar to 400-450 u normally involves gas and liquid chromatography, coupled with mass spectrometry (GC-MS and LC-MS, respectively). However, these techniques cannot readily be adapted to examine samples that contain higher-molecular-mass materials. Chromatographic and mass spectrometric methods are often limited by the volatility of the samples, while liquid chromatographic methods may be limited by solubility in the solvents used. Materials of higher mass are characterized using methods that have been developed to overcome the limitations imposed by volatility and solubility in common solvents. As outlined in the text, they have been the subject of some debate. Much of the work that indicates upper mass limits of similar to 1000-1500 u for coal tars, pitches, and petroleum asphaltenes can be explained in terms of limitations of the particular analytical techniques. The new emphasis on characterizing increasingly heavier materials grows out of a need in oil refineries and elsewhere, for fresh ideas about processing higher-mass feedstocks. Currently, above the similar to 450-500 u range, no single method is unambiguously capable of indicating molecular mass distributions or chemical structural features in complex fuel-derived mixtures. Advances in this field require a comparison of evidence from several independent analytical methods. This review is mainly focused on the results from size exclusion chromatography (SEC), laser-desorption mass spectroscopy (LD-MS) and matrix-assisted laser desorption/ionization mass spectroscopy (MALDI-MS). SEC, using 1-methyl-2-pyrrolidinone (NMP) as an eluent, has shown agreement with LD-MS and MALDI-MS up to similar to 3000 u and to within a factor of 2-2.5 at up to 15 000 u. Suggestions that the samples formed aggregates have been investigated. There is no confirmable experimental evidence, either from our work or in the literature, showing that aggregation occurs under the dilute conditions prevailing during SEC, using NMP as an eluent.