There seems little doubt that the compilation of papers and discussion at the Symposium on Composition of Petroleum Oils, Determination and Evaluation, held under the auspices of Research Division IV on Hydrocarbon Analysis of ASTM Committee D-2 on Petroleum Products and Lubricants, in New Orleans on February 8 and 9, 1957 and presented here in an ASTM Special Technical Publication, represents the most far-reaching and thorough covering of the general subject of the symposium that has ever been assembled. To those who can remember back to about 1925, the enormous advances in our knowledge of the less volatile liquids of petroleum are very evident. To the younger men who have entered the field in its more advanced stages, this tremendous change over the last 30 years may not be so apparent. For that reason, the writer of this preface is quoting below a paragraph written in 1927 and published in 1928: “From the foregoing discussion of the studies on the less volatile liquids of petroleum very few definite conclusions can be made. Apparently the carbon atoms in the compounds constituting these oils are not saturated with hydrogen—about the only statement on which virtually all investigators agree. Moreover, the results shown by the research on these oils indicate that there are two predominating groups. In the group termed ‘saturated,’ for lack of a better name, the unsaturation of the carbon atom is taken up by ring formation; in the other, or the ‘unsaturated group,’ at least a part of the unsaturation is due to double-bonded side chains, bridged structure such as is exhibited by the terpenes, or open chain hydrocarbon with conjugated double bonds. The structures of these groups have never been proved and can only be surmised from the physical and chemical properties and from analysis. There is a marked difference in the properties of the two groups, and most investigators are in general agreement in correlation of these properties with the groups. In general, the saturated group, which may be supposed to consist of polynuclear naphthenic hydrocarbons, is relatively stable and inactive and is slightly soluble in organic solvents and concentrated sulfuric acid. It usually has a lower carbon-hydrogen ratio than the unsaturated group and does not unite readily with iodine and bromine. This small addition of iodine and bromine points to a ring structure as the nucleus of the unsaturated hydrocarbons. The viscosities, specific gravities, and indices of refraction have, in general, been found to be higher in the unsaturated compounds.” Compare this picture with the present situation as revealed by this symposium. The only separative tools of consequence 30 years ago were vacuum distillation—and not high-vacuum, either—and embryonic solvent extraction. Today, distillation is still one of our most useful tools, but it has been developed to the point where it now includes high-vacuum distillation and molecular, extractive, and azeotropic distillation. Solvent extraction has been developed commercially, but it still may have potentialities in the laboratory, and the possibility of applying solvents as an analytical tool is brought out in one of the papers of this symposium. In addition to these older tools, the newer methods of absorption on activated solids, such as silica gel and alumina, and thermal diffusion have appeared. Although there has been great development of the adsorptive technique in many directions, it is still a dynamic methodthat has not yet reached its ultimate development, as witnessed by the recent surge in the use of gas-liquid chromatography. A study of the papers in this symposium relating to thermal diffusion reveals that, although much progress has been made, this method is still in its infancy, but its future is very bright. Also, chemical reactions are now being used to effect separations. Such reactions as the formation of urea and thiourea complexes, sulfonation, hydrogenation, and dehydrogenation are all standard means of effecting certain types of separations in the laboratories of today. For the most part these were unknown, or at least unapplied, in the early days. Thus, giant steps have been taken in developing and using new methods of separation and, to a considerable extent, our advances have been made possible through these new means.