An on-line method is presented for simultaneous size distribution and particle density measurement, based on parallel measurements made by SMPS and ELPI. The measured SMPS number distribution is integrated with the ELPI response functions to produce calculated current response. By varying the density value, the best fit is sought between the calculated and measured current response. Simulation tests made showed relatively good stability against small uncertainties in the distribution and the response functions. Test measurements of well-defined aerosols with known density were conducted. Density values of 0.86, 1.1, and 1.9 g/cm(3) were measured for liquid particles of DOS, Santovac vacuum oil, and Fomblin vacuum oil, respectively. These. values are within 8% of the accepted bulk values. For solid particles of NaCl, Zn, and Ag, slightly larger experimental errors in the range of 4-18.2% were found. (C) 2002 Elsevier Science Ltd. All rights reserved.