This paper presents an experimental investigation on the number concentration decay and size distribution of polydisperse diesel exhaust particles and paper ash particles in a 1.6 m(3) chamber under still conditions and stirred conditions. The particles were measured with two instruments thus allowing particles from 0.016 to 20 mu m to be investigated. Initial number concentrations were above 2 x 10(11) m(-3) to allow simultaneous coagulation and deposition of the particles. The experimental results show that both still and stirred conditions are effective in reducing particles smaller than 0.04 mu m in diameter while stirring additionally reduces the concentration of particles above 0.6 mu m. A regression method was applied to derive the coagulation coefficients and the deposition velocities from the experimental data. The derived values were in the same range of values as those reported in literature. A statistical analysis shows that there is no significant difference between the coagulation coefficient for still and stirred conditions but the deposition velocity is significantly higher under stir-red conditions. Further analysis shows that under still conditions coagulation dominates the concentration decay with over 80% contribution to the total loss rate, whereas under stirred conditions the contribution of coagulation decreases to as low as 20%, after a period of 1500 s. (C) 2006 Elsevier Ltd. All rights reserved.