Ion-molecule reactions within ionized methane clusters were investigated utilizing electron ionization mass spectrometry, A number of ionic reaction products were observed and interpreted as series of cluster ions of the form (CH4)(n) - X+ where X = CH4, CH5, C2H2, C2H3, C2H4, C2H5, C2H6 and C2H7 and n greater than or equal to 0. These cluster series, except for X = C2H7 and C2H6, are interpreted in terms of known bimolecular ionic reactions with both electron impact energy and pressure dependences. The two cluster ions with X = C2H7 and C2H6 demonstrate cluster reactions occurring easily in a solvated environment. The C3H3+ and C3H5+ ions show evidence of consecutive ionic reactions. Possible reaction paths are discussed and compared with previous work using photoionization of methane clusters and high-pressure mass spectrometry. (C) 1998 John Wiley & Sons, Ltd.