The charge-transfer-reaction between molecular oxygen ions (O2 +) and methyl iodide (CH3I) is studied to investigate if consistent environmental quantification of the gas phase CH3I is possible without prior calibration. The neutral CH3I molecule was chosen because this compound is of atmospheric chemistry and environmental importance in the field of nuclear power plant safety and nuclear energy. Molecular oxygen was used as a reagent ion source in a commercial Proton-Transfer-Reaction Time-of-Flight Mass Spectrometer (PTR-ToF-MS) to produce molecular oxygen ions (O2 +). The use of O2 + ions allows for fast, sensitive and specific detection of gas phase CH3I via an electron exchange reaction O2 + + CH3I → CH3I + + O2. The 13 instrument response was linear in the 1-150 ppbv range and its sensitivity was humidity independent. The detection sensitivity of CH3I normalized by the O2 + count rate of 10 6 cps was found to be 15 S = 22.6±0.3 ncps/ppbv, independent of relative humidity. A typical O2 + primary ion signal was (2.0 ± 16 0.2) × 10 6 cps. The lowest measured CH3I concentration was 0.23±0.10 ppb. Error is ±σ. The theoretical collision rate based on the dipole moment and molecular polarizability values is calculated using the Langevin collision rate (kL) approximation, the average-dipole-orientation (ADO) theory and the capture rate coefficient (kCAP) based on trajectory calculations. The experimental rate constant, kexp, for the electron transfer reaction between O2 + ions and CH3I is calculated to be (1.72 ± 0.22) × 10 −9 cm 3 s-1. Listed errors are ± and represent precision only. The experimentally determined value agrees very well with the theoretical collision rate values, L = 1.24 × 10 −9 cm 3 s-1 , ADO = 1.73 × 10 −9 cm 3 s-1 and CAP = 1.48 × 10 −9 cm 3 s-1. The obtained results indicate that the PTR-MS technique is an excellent analytical method to quantify gas phase CH3I.