Mainstream tobacco smoke (MTS) was collected from Camel and Marlboro cigarettes for the determination of the delivery levels and equilibrium gas/particle partitioning constants K(p) (m(3) microg(-)(1)) of 26 volatile organic compounds (VOCs) of toxicological interest. K(p) values are important for understanding the fractional distribution of each compound of interest between the gas and the particle phases of MTS. The experimental method involved (i) drawing a smoke sample into a Teflon sampling bag at 20 degrees C, (ii) allowing the smoke particulate matter (PM) to collect on the walls of the bag, (iii) sampling the bag to determine the initial gas phase concentration of each VOC, (iv) removing the gas phase from the bag, (v) refilling the bag with humidified nitrogen gas, (vi) reestablishing the gas/PM equilibrium, and (vii) redetermining the gas phase concentrations. For each smoke sample, a comparison of the initial and redetermined gas phase concentrations allowed calculation of the total (i.e., gas + particle) delivery level (= m(tot), ng cig(-)(1)) and K(p) value (= c(p)/c(g)) at 20 degrees C for each compound, where c(p) (ng microg(-)(1)) = concentration in the PM phase and c(g) (ng m(-)(3)) = concentration in the gas phase. Significant deliveries were observed for a number of carcinogenic VOCs. For the Camel cigarettes tested, the average m(tot) values for 1,3-butadiene, acrylonitrile, and benzene were 10(4.6), 10(4.4), and 10(4.8) ng cig(-)(1), respectively; for Marlboro, the m(tot) values were 10(5.0), 10(4.6), and 10(4.7) ng cig(-)(1), respectively. For each of the 26 VOCs, the smoke PM from the two brands yielded very similar K(p) values at 20 degrees C. In addition, the vapor pressure-dependent K(p) values of the 26 VOCs were in close agreement with predictions made by the Pankow theory of absorptive gas/particle partitioning [Pankow, J. F. (1994) Atmos. Environ. 28, 185-188]. These results can be used in general predictions of chemical behavior in tobacco smoke, including deposition mechanisms and rates in the respiratory tract from inhaled MTS. Example calculations are provided to illustrate how the gas phase fraction at equilibrium (f(g,e)) increases strongly with increasing compound vapor pressure and temperature and with dilution of the inhaled tobacco smoke total PM concentration (microg m(-)(3)).