Abstract A modified Finnigan LCQ quadrupole ion trap has been used to determine the equilibrium constant of the complexation reaction of thiophenolate with 2,2,2-trifluoroethanol. The process is particularly useful as a thermometer reaction because it has an exceptionally large temperature dependence. Using literature values for the thermochemistry, an effective ion temperature of 310 ± 20 K is indicated for the ion trap. This value is much lower than some earlier estimates for ion traps, but is consistent with a recent theoretical analysis and some previous interpretations of experimental data. The results suggest that quadrupole ion traps are suitable for studying gas phase reactions under nearly thermal conditions.