In this work, the proton transfer reaction between the hydronium ion (H 3 O +) and methyl iodide (CH 3 I) is studied to investigate if consistent quantification of the gas phase CH 3 I is possible in humid air. The neutral CH 3 I molecule was chosen because this compound is of environmental importance in the field of nuclear power plant safety and nuclear energy. Water was used as a reagent ion source in a conventional Insbruck PTR-MS to produce H 3 O + reagent ions. The use of H 3 O + ions allows for fast, sensitive and specific detection of gas phase CH 3 I via a proton-transfer reaction H 3 O + + CH 3 I → [CH 3 I-H] + + H 2 O. The instrument response was linear in the tested 5 to 96 ppbV range and the PTR-MS sensitivity was observed to be humidity dependent. The observed sensitivity was in found to range between 1.6 to 3.3 cps/ppb at relative humidity between 63 and 15% at T= 23 • C. A typical H 3 O + primary ion signal was 10 7 cps and the normalized sensitivity was in the range between 0.16 and 0.33 ncps/ppb. The instrument CH 3 IH + ion background rate was 6.8 ± 1.4 cps and the dwell time was 1 second. The detection limit was calculated as 3 times the standard deviation of the background level and ranged between 1.3 and 3.8 ppb. The theoretical collision rate based on the dipole moment and molecular polarizability is calculated. The theoretical collision rate is compared with the experimentally obtained values. The results indicate that the PTR-MS technique is a good analytical method to detect and quantify gas phase CH 3 I concentrations.