In gas phase ion chemistry, the growth of larger molecules is known to occur through association of ions and neutrals. Where the ion attaches to the neutral is important because it can influence the possibility of additional associations, effectively enabling or terminating further molecular growth. This was investigated by using a Selected Ion Flow Tube (SIFT) at 300 K to study the reactions of CH3+ with the following series of single-ring homocyclic and heterocyclic molecules: benzene (C6H6), cyclohexane (C6H12), pyridine (CAN), pyrimidine (C4H4N2), piperidine (CAIN), 1,4-dioxane (C4H8O2), furan (C4H4O), pyrrole (CAN), and pyrrolidine (CAN). Most of the reactions, except for 1,4-dioxane, pyrrole, and pyrrolidine, proceed at the gas kinetic rate. In the ion product distributions, charge transfer, hydride ion abstraction, proton transfer, fragmentation, and association were observed. In particular, proton transfer is seen to be small in all cases even though these channels are energetically favorable. Association is appreciable when the molecules are aromatic (except for furan) and nonexistent when there are no pi electrons in the ring. CH3+ ions are an important intermediate in molecular synthesis in interstellar clouds and in the Titan ionosphere and ring molecules have also been detected in these media. The significance of the studied reactions to these media is discussed.