Many concepts of premixed diesel combustion at reduced temperatures have been investigated over the last decade as a means to simultaneously decrease engine-out particle and oxide of nitrogen (NO) emissions. To overcome the trade-off between simultaneously low particle and NO emissions versus high "diesel-like" combustion efficiency, a new dual-fuel technique called Reactivity Controlled Compression Ignition (RCCI) has been researched. In the present study, particle size distributions were measured from RCCI for four gasoline:diesel compositions from 65%:35% to 84%:16%, respectively. Previously, fuel blending (reactivity control) had been carried out by a port fuel injection of the higher volatility fuel and a direct in-cylinder injection of the lower volatility fuel. With a recent mechanical upgrade, it was possible to perform injections of both fuels directly into the combustion chamber. Particle size distributions were measured at four different gasoline injection timings for each gasoline:diesel fuel reactivity blend, while the ignition-controlling diesel injection timings remained constant. Increased particle mass and number emissions were measured for increased diesel fueling and advanced in-cylinder gasoline injection timing (especially from -340 to -360° aTDC). In addition, effects of heated primary dilution ratio on the particle size distribution from one of the "standard" RCCI engine operating conditions was measured to provide some information of the particles' volatility and how high of dilution ratio would be necessary to ensure stable measurements.