The steady-state effective particle density of mainstream smoke from a University of Kentucky 3R4F reference cigarette was determined using a Differential Mobility Analyser (DMA) and Centrifugal Particle Mass Analyser (CPMA). The cigarette smoke was generated using a smoking machine under ISO puffing parameters (35 ml puff of 2 s duration, every 60 s) and collected in a Tedlar (R) bag. This smoke generation process resulted in the first puff of the smoking cycle aging approximately 7 min longer than the last puff. The effective particle density (measured immediately upon completion of the last puff) was found to be independent of the particle mobility size, indicating the particles have a spherical morphology, with an average density of 1180 +/- 113 kg/m(3). Particle coagulation was also found to occur within the Tedlar (R) bag by comparing particle mobility size distributions, measured with a Scanning Mobility Particle Sizer (SMPS), against an analytical model. This model showed that particle coagulation dominated the particle number concentration decay within the Tedlar (R) bag compared to particle diffusion or settling losses. Therefore cigarette smoke particles must have a liquid component to maintain a constant effective particle density function in the presence of coagulation. After the 7 min filling process, the effects of particle aging time and initial particle number concentration in the Tedlar (R) bag on the effective particle density were found to be small and indistinguishable within the bias uncertainty of the measurement system. (C) 2014 The Authors. Published by Elsevier Ltd.