Fuel reformulation represents a major approach to reduce emissions in the current fleet. In this work, a medium-duty engine was used to determine the potential benefits of using reformulated Diesel fuel with controlled composition. This particular engine technology is used approximately in more than 60% of the corresponding transport fleet. The fuel properties that received special attention were sulfur content (390 - 8670 ppm), aromatic content (9.8 - 32.6 wt %), cetane number (40 - 51.6) and distillation temperature for 90 % volume recovered T90 (307 - 350 °C). Testing was conducted on the engine installed on a test bench and operated on a 4-mode cycle with two speeds and medium to high load. Data was taken on each mode and statistical procedures for averaging, weighting and analyzing, were applied to study engine operation. Combustion performance was observed based on a heat release analysis, comparing burning rates and ignition delay. Exhaust emissions of HC, CO, NOx and specific fuel consumption were determined. Particulates were measured using a mini-dilution tunnel technique using two separate filters for each measuring condition. Chemical analysis and detailed fuel characterization is used to explain the emissions results and the effects of fuel composition on emissions. The steady state testing allowed discrimination between fuel formulations. The absolute emissions and calculated reductions, were weighted using assumed service factors, indicating the importance of properly determining representative engine operating conditions for fuel comparisons. Hydrocarbons and CO were dependent on aromatic content with reductions as high as 11 % and 12 % respectively. NOx is linearly dependent on aromatic content in the conditions studied with reductions as high as 8 %. Specific fuel consumption can be reduced as much as 2.2 % due to increased combustion efficiency. The combined effects of aromatics, sulfur content and T90 on particulate emission rates was very dependent on the operating condition, these were similar for all fuels for higher fueling rates, where in-cylinder conditions favor soot oxidation.