The influence of dilution condition is known to affect the particle number size measurements of engine exhaust samples. However, it is preferable to understand how the dynamics of mixing and cooling controls the dilution scheme, rather than the dilution ratio alone as is commonly used. In this study, the effect of mixing and temperature of dilution gas on exhaust samples in a mixing-tube diluter was explored for two engine load conditions. The observed global trends of the particle number concentrations (PNC) using the mixing-tube diluter (MTD) are consistent with the findings published with different dilution systems. Relative to the two operating conditions, it was observed that, the PNC in the sub 30nm diameter were greater during the lower load operation compared to the higher load at all dilution ratios and dilution gas temperatures. Particles from the lower engine load operation were viewed to have more volatile fractions, compared to those measured under the higher load operation. The behaviour of volatile fractions upon dilution was used to study the effect of cooling and mixing of aerosol. This behaviour was found to be dependent upon the saturation characteristics of the volatiles. The characteristic of saturation ratio (S) as used to express the effect of dilution on raw exhaust, initially increases with dilution ratio before reaching a maximum and declining. This was evident when both particle number and mass concentrations were considered. It demonstrates that there is an intermediate range of dilution conditions that favour condensation of volatile fractions.