A 1.9 liter Volkswagen TDI engine has been modified to accomodate the addition of hydrogen into the intake manifold via timed port fuel injection. Engine out particulate matter and the emissions of oxides of nitrogen were investigated. Two fuels,low sulfur diesel fuel (BP50) and soy methyl ester (SME) biodiesel (B99), were tested with supplemental hydrogen fueling. Three test conditions were selected to represent a range of engine operating modes. The tests were executed at 20, 40, and 60 % rated load with a constant engine speed o 1700 RPM. At each test condition the percentage of power from hydrogen energy was varied from 0 to 40 %. This corresponds to hydrogen flow rates ranging from 7 to 85 liters per minute. Particulate matter (PM) emissions were measured using a scaning mobility particle sizer (SMPS) and a two stage micro dilution system. Oxides of nitrogen were also monitored. For most conditions of both diesel and biodiesel testing a reduction in total mass and umber PM emissions is observed with increasing amounts of hydrogen energy input. A small reduction of NO x emissions is observed with 5% hydrogen energy input at all load conditions. At all other conditions NOx emissions show little change with hydrogen energy input. At all conditions tested there is a significant increase in the ratio of NO2 to NOx in the engine out emissions with increasing amounts of hydrogen. The most notable changes occur with less than 10% hydrogen energy added. This phenomenon is seen with both diesel and biodiesel.