Diesel low temperature combustion (LTC) is an engine operational strategy that effectively reduces soot and oxides of nitrogen (NOx) emissions. A downside to LTC is particulate matter (PM) containing high levels of semi-volatile material due to elevated hydrocarbon concentrations. The mechanics of formation and composition of this semi-volatile PM has been largely uncharacterized. In this work, we investigate engine-out PM and HC emissions from LTC modes and equivalent conventional diesel combustion modes. An experimental strategy using tandem differential mobility analysis was implemented to investigate organic PM volatility as a function of particle size. The results indicate that semi-volatile PM from LTC is comprised of compounds spanning a wide range of volatilities. Differences in PM volatility were even greater for particles of different size groups. The results show that gas-to-PM conversion processes require additional investigation to determine the clear impact of LTC implementation on real-world PM emissions.