A method is presented to predict the soot volume fraction in soot-laden gas streams in systems where thermophoresis is the dominant mechanism of particle deposition onto adjoining surfaces. In particular, we considered deposition of silica particles on a circular cylinder in cross-flow to a premix CH4/O-2 flame, a setup similar to the one used in the outside vapor deposition process used for making optical fibers. Silica particles were produced by introducing SiCL4 along with the premix gases to the burner and were collected on a cylinder. Heat flux and mass deposition rate measurements on the cylinder were performed and recorded as a function of time. Considering thermophoresis to be the dominant mechanism of particle deposition, a simple theory was developed to establish the relationship between the measured quantities. The theory predicted that the thickness at any given time t was expected to increase linearly with the integral of q"(t) dt (integrated from t = 0 to t = t), where q"(t) is. the heat flux. Such a linear relationship was observed for five different reactant flow rates confirming thermophoresis to be the dominant mechanism of particle deposition. Soot volume fraction and soot mass fraction were calculated from the slope of these linear fits and were seen to be in good agreement with the estimates of the soot fraction from light scattering measurements. Based on the light scattering estimates of particle diameter, particle number densities were also estimated. (C) 2003 Elsevier Science Ltd. All rights reserved.