Gasoline Compression Ignition (GCI) engines have the potential to achieve high fuel efficiency and to significantly reduce both NOx and particulate matter (PM) emissions by operating under dilute, partially-premixed conditions. This low temperature combustion strategy is dependent upon direct-injection of gasoline during the compression stroke and potentially near top dead center (TDC). The timing and duration of the in-cylinder injections can be tailored based on speed and load to create optimized conditions that result in a stable combustion. We present the results of advanced aerosol analysis methods that have been used for detailed real-time characterization of PM emitted from a single-cylinder GCI engine operated at different speed, load, timing, and number and duration of near-TDC fuel injections. PM characterization included measurements of size and composition of individual particles sampled directly from the exhaust and after mass and/or mobility classification. We use these data to calculate particle effective density, fractal dimension, dynamic shape factors in free-molecular and transition flow regimes, average diameter of primary spherules, number of spherules, and void fraction of soot agglomerates. The data indicate that the properties of GCI particulates varied markedly depending upon engine load. Under low-load conditions (5.5 bar net Indicated Mean Effective Pressure, IMEP), PM is comprised of a mixture of particles similar to 70% of which are compact organic particles and similar to 30% are fractal soot aggregates. The soot aggregates have fractal dimension of 2.11, are constructed of primary spherules with average diameter of 40 nm, and composed of elemental and organic carbon at similar to 55:43 ratio by weight. Under high-load conditions (14 bar net IMEP), all the particles are fractal soot agglomerates with nearly identical fractal dimension and composition, but constructed of primary spherules with average diameter of 26 nm. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.