Rationale The volatile compounds generated by the electrochemical reduction of atmospheric carbon dioxide and nitrogen include isobaric methanol (CH 3 OH) and, potentially, hydrazine (N 2 H 4 ). To achieve quantification of hydrazine molecules by selected ion flow tube mass spectrometry (SIFT‐MS), its reactions with H 3 O + , NO + and O 2 + reagent ions must be understood. Methods A SIFT study (using a SIFT‐MS instrument) was carried out to obtain rate coefficients and product ions for the reactions of H 3 O + , NO + and O 2 + reagent ions with N 2 H 4 and CH 3 OH molecules present in the humid headspace of their aqueous solutions. Using the kinetics data obtained, solution headspace concentrations were determined for both compounds as a function of their liquid‐phase concentrations at 10, 20 and 35°C. Results Both compounds react with H 3 O + ions via rapid proton transfer to produce CH 3 OH 2 + and H 5 N 2 + ions with the common m / z value of 33. It is revealed that NO + rapidly transfers charge to N 2 H 4 (rate coefficient k = 2.3 × 10 −9 cm 3 s −1 ) but only slowly associates with CH 3 OH ( k 2 eff = 7.1 × 10 −11 cm 3 s −1 ). Thus, selective analysis can be achieved using both H 3 O + and NO + reagent ions. The headspace methanol vapour concentration was found to increase with increasing solution temperature, but that of hydrazine decreased with an associated increase of ammonia (NH 3 ) as measured with O 2 + reagent ions. Conclusions The isobaric compounds methanol and hydrazine can be separately analysed in real time by SIFT‐MS using H 3 O + and NO + reagent ions, even when they co‐occur in humid air. The evolution of hydrazine from aqueous solutions can be quantitatively monitored together with its decomposition at elevated temperatures.