Low-energy collision-induced dissociation (CID) and ion-molecule reactions with 2-methyl-1,3-dioxolane (MD) performed by pentaquadrupole (QqQqQ) mass spectrometry were applied to locate the charge site in isomeric heteroaromatic cations. The 2-, 3-, and 4-pyridyl cations are indistinguishable by CID. However, as suggest ed by MS3 experiments and ab initio calculations, the 2-pyridyl cation reacts extensively with MD by a transacetalization-like mechanism to afford a bicyclic dihydrooxazolopyridyl cation. The 3- and 4-pyridyl cations, on the contrary react predominantly with MD by proton transfer, as does the analogous phenyl cation. The 2-. 4-, and 5-pyrimidyl cations display characteristic CID behavior. In addition, the 2-pyrimidyl cation reacts extensively with MD by the transacetalization-like mechanism, whereas proton transfer occurs predominantly for the 4- and 5-pyrimidyl cations. The ions thought to be the 2- and 3-furanyl and 2- and 3-thiophenyl cations show indistinguishable CID and ion-molecule behavior. This is most likely the result of their inherent instability in the gas phase and their spontaneous isomerization to the corresponding butynoyl and butynethioyl cations HC=CHCH2C=O+ and HC=CHCH2C=S+. These isomerizations, which are considerably exothermic according to G2(MP2) ab initio calculations, are indicated by a series of experimental results. The ions dissociate upon CID by loss of CO or CS and undergo transacetalization with MD. Most informative is the participation of HC=CHCH2C=S+ in a transacetalization/dissociation sequence with replacement of sulfur by oxygen, which is structurally diagnostic for thioacylium ions. It is therefore possible to locate the charge site of the 2-pyridyl and the three 2-, 4-, and 5-pyrimidyl cations and to identify the isomeric precursors from which they are derived. However, rapid isomerization to the common HC=CHCH2-C=O(S)(+) ion eliminates characteristic chemical behavior that could result from different charge locations in the heteroaromatic 2- and 3-furanyl and 2- and 3-thiophenyl cations.4711611168