The objective of this work is to improve the understanding of variables like dilution and sampling conditions that contribute to particle-based emission measurements by assessing and comparing the nucleation tendency of Diesel aerosols when diluted with a porous wall dilutor or an air ejector in a laboratory setting. An air-ejector dilutor and typical dilution conditions were used to establish the baseline sensitivity to dilution conditions for the given engine operating condition. A porous tube dilutor was designed and special attention was given to integrating the dilutor with the exhaust pipe and residence time chamber. Results from this system were compared with the ejector dilutor. Exhaust aerosols were generated by a Deere 4045 Diesel engine running at low speed (1400 rpm) and low load (50 Nm, ~10% of rated). Primary dilution parameters that were varied included dilution air temperature (25 and 47°C) and dilution ratio (5, 14, and 55). Particle measurements were made at 0.3, 0.75, and 1.0 s to evaluate particle growth in the residence time chamber. Exhaust size distribution measurements made using the ejector dilutor were bimodal with high concentrations of nucleation mode particles. Varying the dilution ratio from 5 to 55:1 (with a dilution air temperature of 25°C and residence time of 1 s) led to the greatest change in the particle number concentration (4 × 108 to 4 × 1010 particles/cm3) compared to changes in the other variables. Particle concentration was lower with higher dilution air temperatures and particles were larger in size. Size distributions downstream of the porous tube and ejector dilutor were qualitatively similar in shape. Using a simple dilution model and equations for particle growth in the free molecular regime, particle growth in the two residence time chambers was compared. Model results suggest that dilution in the porous tube dilution system occurs more slowly than in the ejector dilutor. This is consistent with the findings that the particle number concentrations were consistently higher and the geometric mean diameter was generally 1 to 5 nm larger downstream of the porous tube dilutor.