Atmospheric pressure mass spectrometry (APCI-MS) was used to investigate the positive ions in air containing acetone (A), methanol (M) and mixtures thereof (A + M), subjected to +dc corona discharges. The results of experiments with isotopically labelled analogues, perdeuterated acetone A(deu) and methanol M-deu, and relevant thermochemical data found in the literature allowed us to identify the main ionic reactions occurring in single component systems (A or M) and in binary mixtures (A + M). It is concluded that, thanks to its significantly higher proton affinity, A is very efficient in quenching M-derived ions at atmospheric pressure. These conclusions provide a rationale for interpreting the results of a parallel investigation on the reciprocal effects of M and A when treated together in air at atmospheric pressure with +dc corona in a non-thermal plasma reactor developed previously in our laboratory. Specifically, we observed a marked drop in the degradation efficiency of methanol when it was treated in the presence of an equivalent amount of acetone. This effect is attributed to acetone interfering with ion-initiated degradation processes of methanol, and supports the conclusion that ions and ionic reactions are important in dc+ corona induced oxidation of volatile organic pollutants in air.