Rate constants and branching fractions for the reaction Of O-2(+) with C8H10 (ethylbenzene) have been measured in the recently upgraded turbulent ion flow tube (TIFT) and are reported here as a function of temperature from 423 to 573 K and number density from 3 x 10(17) to 25 x 10(17) molecules cm(-3). The results reported here represent the first measurements to be made on this instrument as a function of both temperature and pressure. The rate constants for the reaction Of O-2(+) with C8H10 are collisional and exhibit no appreciable variation with temperature or pressure. The reaction proceeds primarily by dissociative and nondissociative charge transfer forming two main product ions, namely, C8H10+ and C7H7+. The ratio Of [C8H10+] to [C7H7](+) depends strongly on both the buffer gas number density and the temperature. Increasing the number density increases the abundance Of C8H10+, indicating that collisional stabilization of the charge transfer excited state is occurring. Measurements were made using both He and N-2 buffers. Nitrogen is almost twice as efficient at stabilizing the charge transfer product than is He. A simple model predicts the ratio of [C8H10+] to [C7H7+] to depend linearly on number density, which is consistent with the experimental results. At a constant number density, the abundance Of C7H7+ increases with temperature. A small amount of thermal dissociation of C8H10+ has been observed at 573 K.