Aldehyde and unburned fuel emissions (UBF) were measured from experimental 6V-92TA and 6V-71N methanol-fueled heavy-duty diesel engines. The 6V-71N engine was tested with experimental oxidation catalytic converters.Formaldehyde accounted for 97-99% of the total aldehydes emitted from both engines. Aldehyde and unburned fuel concentrations were highest in the idle modes of the 13-mode test and decreased with increasing engine speed and load. The 13-mode weighted aldehyde and UBF emissions for the 6V-92TA engine were 0.10 G/BHP-HR and 2.01 G/BHP-HR, respectively. The 13-mode aldehyde and UBF emissions from the 6V-71N engine before catalytic treatment were 0.32 G/BHP-HR and 4.51 G/BHP-HR, respectively. These emissions were reduced to 0.19 G/BHP-HR and 1.10 G/BHP-HR with catalytic treatment.Total aldehyde emissions from the methanol-fueled engines compared favorably with the 0.07 to 0.28 G/BHP-HR range reported for heavy-duty conventionally fueld diesel engines. Formaldehyde emissions from these engines were higher than those of conventionally fueled diesel engines but similar to those of other methanol-fueled diesel engines.Particulate and NOx emissions from these engines were very low and ranged from 0.02-0.22 G/BHP-HR and 1.45-1.66 G/BHP-HR, respectively. These engines show promise for meeting the 1991 urban bus 0.10 G/BHP-HR particulate standard.