Increasingly stringent emissions regulations, rising oil prices, and an increased focus on environmental awareness are driving the search for clean, alternative fuels. Derived from natural gas, coal, and even biomass Fischer-Tropsch (FT) fuels are one such alternative. The inherently clean nature of FT diesel coupled with the fact that FT diesel exhibits similar physical properties to those of conventional diesel make FT diesel an ideal candidate as both a blending agent with and eventual replacement for conventional petroleum-based diesel fuels.The potential for emissions reduction with FT diesel fuels in laboratory engine tests and on-road vehicle tests is well documented. While a number of chemical and physical characteristics of FT fuels have been attributed to the observed reduction in emissions, the actual effects of both the fuel properties and in-cylinder combustion characteristics in modern diesel engines are still not well understood. In this study a 2002, six-cylinder, 5.9 liter, Cummins ISB 300 diesel engine, outfitted with an in-cylinder pressure transducer, was subjected to a subset of the Euro III 13-mode test cycle under steadystate operating conditions. Emissions and in-cylinder pressure measurements were conducted for neat FT diesel, low sulfur diesel (LSD), ultra-low sulfur diesel (ULSD), and a blend of FT/LSD. The experimental results show a significant reduction in regulated emissions with the neat FT fuel and a more than proportional reduction in particulate emissions for the blend. In order to provide further insight into the emissions behavior of the fuels, combustion characteristics were determined from a heat release analysis based on the in-cylinder pressure measurements. In addition, a detailed chemical analysis of the fuels and particulate emissions was carried out. The differences in the measured combustion characteristics and fuel properties were compared to the emissions variations between the fuels studied, and an explanation for the observed emissions behavior of the fuels was developed.