The effects of fuel sulfur concentration on heavy-duty diesel emissions have been studied at two EPA steady-state operating conditions, mode 9 (1900 RPM, 75% Load) and mode 11(1900 RPM, 25% Load). Data were obtained using one fuel at two sulfur levels (Low Sulfur, LS = 0.01 wt% S and Doped Low Sulfur DS = 0.29 wt% S). All tests were conducted using a Cummins LTA10-300 heavy-duty diesel engine.No significant changes were found for the nitrogen oxides (NOx), soluble organic fractions (SOF) and XAD-2 (a copolymer of styrene and divinylbenzene) organic component (XOC) due to the fuel sulfur level increase at either engine mode. The hydrocarbon (HC) levels were not significantly affected by sulfur at mode 9; however, at mode 11 the HC levels were reduced by 16%. The total particulate matter (TPM) levels increased by 17% at mode 11 and by 24% at mode 9 (both significantly different). The solid (SOL) emissions were not affected by fuel sulfur content at mode 11, but at mode 9 the data set indicated a 30% increase in the SOL emissions with the fuel sulfur increase. The sulfate (SO4 =) levels, which were significantly different for both mode and fuel, increased from 0.02 mg/m3 to 1.28 mg/m3 and from 0.08 mg/m3 to 1.83 mg/m3 at mode 11 and 9, respectively.Particle size distribution results indicated that the two fuels had nearly identical accumulation modes; however, with the low sulfur fuel the smaller nuclei-mode particles virtually disappeared. Several significant differences in polycyclic aromatic hydrocarbon (PAH) emissions were found between the two fuel sulfur levels with higher PAH levels usually associated with low sulfur fuel. For the two engine modes tested, more significant difference were seen between fuels for total PAH (SOF + XOC levels) for the lower temperature mode (mode 11). The PAH partitioning between the soluble organic fraction and the XOC was found (in most cases) to follow linear isotherm adsorption theory and this information was used to predict levels of PAH below detection limits. Mutagenic activity generally decreased with increasing fuel sulfur concentration.