The objective of this research was to obtain diesel particle size distributions from a 1988 and a 1991 diesel engine using three different fuels and two exhaust control technologies (a ceramic particle trap and an oxidation catalytic converter). The particle size distributions from both engines were used to develop models to estimate the composition of the individual size particles. Nucleation theory of the H2O and H2SO4 vapor is used to predict when nuclei-mode particles will form in the dilution tunnel. Combining the theory with the experimental data, the conditions necessary in the dilution tunnel for particle formation are predicted. The paper also contains a discussion on the differences between the 1988 and 1991 engine's particle size distributions.The results indicated that nuclei mode particles (0.0075-0.046 μm) are formed in the dilution tunnel and consist of more than 80% H2O-H2SO4 particles when using the 1988 engine and 0.29 wt% sulfur fuel. Nucleation theory indicated that H2O-H2SO4 particles may form during dilution at 0.03 wt % fuel sulfur levels and above. The 1991 engine was designed for lower particulate emissions than the 1988 engine and the 1991 engine's accumulation mode particles (0.046-1.0 μm) were reduced more that 80% by volume compared to the 1988 engine using the same low sulfur fuel. The particle size composition model indicated that using low sulfur fuel and the 1991 engine, the nuclei mode contained more than 45% of the total solid particles and over 85% of the soluble organic fraction.