SCOPE OF PROGRAM The objective of this program has been to conduct an intensive critical review of the state of the art of the nature and photochemical smog reactivity of diesel engine exhaust organic components and to identify needed future research based on this review. In carrying out this review@ both manual and computer searching of published literature was employed along with a review of the personal files of atmospheric sciences and mechanical engineering staff members at Battelle- Columbus. In addition@ a letter requesting information on diesel fuel/exhaust composition and photochemical smog reactivity was sent to over sixty key scientists in the field. The mailing list is Incorporated as the Appendix of this report. Sixty-six percent of the individuals contacted either responded directly or had an associate respond to our request for information. On the whole@ the responses to our request were quite negative; as suspected@ very little work has been done in this area. Based on the responses to our letter canvass@ telephone calls were made to those individuals appearing to possess useful data. In some cases@ these telephone calls were followed by site visits. The results of this state-of-the-art review are presented in the following sections of this report.