This project dealt with the molecular composition of asphalts and with the changes in asphalt composition that have been found to result from the inclusion of aggregate, additives, fillers and extenders in bituminous paving mixtures under normal production conditions in Montana. The method used in this project was high pressure liquid chromatography in the gel permeation mode. Changes in the molecular size distribution (MSD) of an asphalt, under carefully controlled laboratory processing, were shown to depend not only upon the specific asphalt, but also upon the aggregate, the additive, the mixing temperature and the time held at that temperature. Consequently, the MSD of the asphalt in a finished pavement cannot be predicted from the MSD of the original asphalt cement alone. A test method was developed, however, to simulate the net effect of plant mix processing, thus yielding an accurate prediction of the final MSD. In a continuing sub-study on asphalt 'aging', the molecular size characteristics of asphalts in highway pavements were shown to undergo only very minor changes after approximately two years. Earlier predictions of pavement performance, based on molecular profiles of asphalts from core samples removed soon after construction, were actually borne out in two cases that were major paving projects. Several recycling agents were shown to consist mainly of very small molecular size materials. When mixed with salvaged bituminous material, they appeared to be inadequate to compensate for, or to otherwise change, excessive amounts of large molecular size (LMS) material often found in the salvaged asphalt. In another common method of recycling, it was found that the MSD of a mixture of asphalts can be estimated from the MSDs of the individual asphalts and their proportions in the mixture. As an example, a salvaged asphalt containing an excessive amount of LMS material may be modified with a virgin soft asphalt (i.e., 200-300 penetration grade asphalt with few LMS components) so that the resulting mixture closely matches the model for high quality asphalt cement.