We report the first in situ size-resolved density measurements of particles produced by premixed charge compression ignition (PCCI) combustion and compare these with conventional diesel exhaust particles. The effective densities (rho(eff)) of size-classified particles were determined by measurements with a differential mobility analyzer (DMA) and an aerosol particle mass analyzer (APM). Particle inherent densities (rho(i)) were calculated using an expression for particle mass given by idealized aggregate (IA) theory, transmission electron microscopy (TEM) measurements of primary particle diameter (d(pp)), and a comparison of the measured number of particles in each size class with that predicted by a proposed DMA-APM response function for aggregates. The rho(eff) of POCI and conventional diesel particles were similar over a range of diameters characteristic of their number-size distributions. The rho(eff) were 0.89, 0.58, and 0.51 g/cm(3) for conventional diesel and 0.90, 0.62, and 0.42 g/cm(3) for PCCI particles with 50, 100, and 150 nm electrical mobility diameters (d(m)), respectively. The error associated with rho(eff) was about one percent of each measurement. The lowest rho(eff) were observed for exhaust gas recirculation (EGR) levels somewhat lower than that required for PCCI operation. The pi of 50 and 100 run conventional diesel particles were 1.22 +/- 0.14 and 1.77 +/- 0.29 g/cm(3), which is in good agreement with previously reported values. PCCI A for these size classes did not differ significantly (1.27 +/- 0.16 and 2.10 +/-0.20 g/cm(3)), suggesting like amounts of adsorbed liquid hydrocarbons. In addition, for 150 nm particles, the PCCI and conventional rho(i) were the same (2.20 +/- 0.34 g/cm(3)). Given the dose density values, we expect that particulate emissions control with diesel particulate filters (DPFs) would not be adversely affected by PCCI particle physical properties.