Temporally resolved laser-induced incandescence (LII) signals are obtained from different size primary particles produced by the diffusion flames of methane, ethane, ethylene, and acetylene. These results represent the first direct comparison between primary particle sizes based on optical measurements and those directly measured through transmission electron microscopy (TEM). Analysis of the data at different detected wavelengths as suggested by theory reveals a nonmonotonic relation with primary particle size as measured by TEM. Two alternative measures of the temporal decay of the signal at a single detected wavelength reveal a correlation with primary particle size within soot aggregates produced by the different flames. Comparison between predicted primary particle size based on the calibrations using the temporal analysis of the LII signal and TEM measurements reveals agreement within the growth region (low axial heights) and very late in the oxidation region (high axial heights) within an ethylene gas-jet diffusion flame. Significant differences exist at intermediate positions. These differences are interpreted as representing the effects of cluster-cluster aggregation within the oxidation region.