An experimental investigation of the sooting structure of diluted methane-oxygen counterflow flames is reported for partial premixing in the following two nonpremixed flame configurations: Case 1: Nonpremixed flame on the oxidizer side of the stagnation plane, Case 2: Nonpremixed flame on the fuel side of the stagnation plane. Effects of both fuel-side and oxidizer-side partial premixing for Cases 1 and 2 were investigated in a low-strain-rate (similar to 6-8 s(-1)) counterflow flame. Computations using OPPDIF code were in excellent agreement with the measured concentrations of major species and [OH]. Distribution of measured soot volume fraction and particle sizes are presented along with measured distributions of C-2 hydrocarbon species. Soot loading can increase or decrease depending on (a) the level of partial premixing, (b) the side of partial premixing (fuel side or oxidizer side), and (c) the nonpremixed flame configuration. Of particular interest is the trend for fuel-side partial premixing of Case 1, where the peak soot loading, the peak soot particle diameter, and the thickness of the soot zone initially decrease and then increase with progressive partial premixing. The trends presented are discussed based on chemical, dilution, and flow-field effects of partial premixing on soot growth in counterflow flames. Unlike previous literature, which focused on soot inception, this work emphasizes the role of partial premixing on soot growth by taking into account the changes in the temperature-time history of soot particulates in addition to the previously reported "chemical" and "dilution" effects. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.