In this paper results are given from single-cylinder, steady-state engine tests using the Texaco Diesel Additive (TDA) as an in-fuel emission reducing agent. The data include NOx, total unburned hydrocarbons, indicated specific fuel consumption, and heat release analysis for one engine speed (1500 RPM) with two different loads (Φ ≈ 0.3, IMEP = 0.654 MPa and Φ ≈ 0.5, IMEP = 1.006 MPa) using the baseline fuel and fuels with one percent and five percent additive by weight.The emissions were measured in the exhaust stream of a modified TACOM-LABECO single cylinder engine. This engine is a 114 mm x 114 mm (4.5″ x 4.5″) open chamber low swirl design with a 110.5 MPa (16,000 psi) peak pressure Bosch injector. The injector has 8 holes, each of 0.2 mm diameter. The intake air was slightly boosted (approximately 171 kPa (25 psia)) and slightly heated (333 K (140 °F)). In previous research on this engine the emissions, including soot, were well documented.Tests of the baseline diesel fuel and fuel with one weight percent (1%) TDA showed no difference between in the emissions for the two fuels. The pressure data, heat release evaluation and all the emission data were indistinguishable between the two fuels. However tests with five percent additive indicated a significant reduction in NOx and no significant change in the other measured emissions or fuel economy. There were slight differences in the normalized heat release results between the two fuels.Further testing in efforts to replicate the initial results have indicated there is large variation in the composition of the additive. This is attributed to variation in the chemical structure introduced by the heterogeneous reactions involved in making the additive molecule. Subsequent experiments with different additive batches have resulted in NOx emissions ranging from an increase to 30 percent reduction. Mini-dilution soot sampling indicates no increase in particulates for the experiments in which the additives were effective in reducing the NOx emissions.