Density functional theory (DFT) B3LYP calculations characterize the structure and stability of the clusters of tropylium ion (Tr + ) coordinated by methanol molecules Tr + (MeOH) n with n=1-7. Methanol molecules are bound together through strong O-H...O type hydrogen bonds, resulting in a cyclic structure when n=<3, and the methanol cluster thus formed coordinates to Tr + through weak C-H...O type hydrogen bonds. Thus, the formation of the Tr + (MeOH) n clusters is mediated by two kinds of hydrogen bonds. Calculated distances of the O-H...O hydrogen bonds lie in the range 1.570-1.991Å (1.712Å in average) while those of the C-H...O hydrogen bonds lie in the range 2.083-2.319Å (2.160Å in average). Mass spectroscopic experiments demonstrated that Tr + (MeOH) 4 is a dominant, magic-numbered species and that Tr + (MeOH) 3 and Tr + (MeOH) 5 are minor [Chemical Communication, (2001) in press]. The experimental result is analyzed from the viewpoint of energetics. The specific size effect on the stability of Tr + (MeOH) n is a direct consequence of the stability of the (MeOH) n fragment itself.