The effect of particle chemical composition on the counting efficiency of a commercially available n-butanol condensation particle counter (CPC) was theoretically investigated. The activation probability of particles soluble in n-butanol or covered by a soluble coating was determined by Kohler theory, whereas the activation of insoluble particles was determined by heterogeneous nucleation theory. The theoretically predicted counting efficiencies were fit to experimental data to infer the n-butanol microscopic contact angle on insoluble particles or the volume of a soluble layer coating the particle. The calculated microscopic contact angles were found to depend on particle chemical composition, particle diameter, and the CPC saturator-to-condenser temperature difference. The average n-butanol microscopic contact angle on diesel exhaust and CAST soot was determined to be 5-10 degrees, on Emery oil particles close to 0 degrees, on thermally pre-treated tetracontane (C(40)H(82)) particles 25 degrees, and on dry sodium chloride particles 15-20 degrees for CPCs operated at a temperature difference of approximately 7 degrees C (low saturation ratios). The counting efficiencies were very sensitive to particle contamination, as determined by the particle generation method and treatment, an effect that could be reproduced by modified Kohler theory. The dependence of the counting efficiency on particle chemical composition was found to be stronger at lower CPC temperature differences. (C) 2010 Elsevier Ltd. All rights reserved.