To determine the effects of ambient conditions on emissions from light-duty diesels, 174 emission tests were conducted on four vehicles. The major program objective was estimation of a factor to correct NOx emissions to a standard humidity. Other emissions and ambient conditions were also examined for relationships by regression analysis.Humidity effects on NOx emissions were found to be substantial and quite predictable. Relationships observed between other emissions and ambient conditions were relatively weak and unpredictable. Several types of NOx-humidity correction factors were estimated and compared to those already used for other vehicle and engine classes.