Ion-molecule reactions of CH₃OH, CH₃OD, CD₃OH and CD₃OD have been investigated using a mass spectrometer. Secondary ions formed by H⁺or D⁺transfer(by or H or D abstraction)were observed, but no secondary ion of the mass number higher than these reaction products was detected. The reaction of parention was measured using the electron energy of slightly higher than the ionization potential of methanol in order to avoid the contribution of daughter ions. The contribution of daughter ions was checked by changing electron energy at constant gas pressure. The results obtained are as follows: 1. The cross section of the ion-molecule reaction by the proton transfer from methyl group of parent ion to neutral molecule is greater than that of proton transfer from hydroxyl group. 2. Ion-molecule reactions by daughter ions which give the same secondary ions as those by parent ion, also can occur. In the case of the daughter ion CH₂OD⁺, the cross section of D⁺transfer from hydroxyl group is greater than that of H⁺transfer from methyl group. 3. An isotope effect for the total cross section, if any, is very small, but an isotope effect between H and D in the same methanol molecule is clearly observed.