This project’s objective was to examine the viability of a soya bean oil derivative, soya methyl ester (methyl ester) and ARB diesel fuel blends as an interim emissions-reduction solution for California’s transit properties as they progressively convert to cleaner technologies and fuels. A Detroit Diesel 6V92TA MUI engine was operated on a blend of two fuels: #2 diesel, subject to October 1, 1993 California Air Resources Board standards (ARB diesel) and methyl ester. Fuel characterization was conducted on ARB diesel, methyl esters from various suppliers and methyl ester/ARB diesel blends. Fuel quality varied with methyl esters from different suppliers. Engine dynamometer tests identified significant trends in exhaust emissions. When compared to the ARB diesel baseline, higher blend percentages of methyl esters led to increased emissions of oxides of nitrogen (6%-10%), carbon dioxide (2%-3%) and soluble particulate matter (19%-35%). Also noted were reductions in total hydrocarbons (16%-32%), carbon monoxide (8%-22%) and insoluble particulate matter (10%-37%). Chassis dynamometer tests showed similar trends in exhaust emissions. Field tests consisted of daily refuelling (using 20/80 and 25/75 methyl ester/ARB fuel blends) and operation of a mass transit passenger vehicle within the Los Angeles basin. These tests were conducted to determine the blends’ effects on engine performance and wear. Drivers’ comments and periodic engine oil analyses indicated no adverse effects. This project demonstrated that 20% soya oil-based methyl esters/ARB diesel blends do not lower emissions to merit utilization by the heavily regulated California mass transit industry. However, the results do indicate that soya oil-derived methyl esters, coupled with known technologies that reduce the soluble fraction of particulate emissions, deserves further exploration as a possible transition fuel option for the Southern California mass transit sector. This project did not develop any new jobs within the Los Angeles basin.