Measurements of tailpipe emissions from a heavy duty engine operating on mixtures of diesel and Soy-Methyl-Ester (SME) fuels were performed. A standard dynamometer and bench emission analyzers were used to analyze engine performance parameters and exhaust emissions. On-line measurements of formaldehyde (HCHO) in the exhaust with a tunable diode laser spectrometer (TDLS) are described. Preliminary results showed that three engine parameters (power, torque, and BSFC) were not affected by the presence of SME up to 30 percent in the mixtures. Pure diesel fuel and mixtures of diesel-SME fuels exhibit similar properties of regulated exhaust emissions (HC NOx and CO). About a 10 percent increase of formaldehyde with a 10 percent increase of SME concentration was found.