Ion-molecule reactions involving H2 and Ha + in the interstellar medium are examined when hydrogen is mainly in molecular form. Most small molecules except H2 and CH+ can be formed as a result of these reactions. For the ionization rate due only to the observed high-energy cosmic rays, the rate of molecule formation is comparable with or exceeds upper limits for previously considered processes at particle densities < 10 cm-a. Due to the widespread abundance of H2, reactions with H2 and Ha + can be the chief formation process for small interstellar molecules in a large fraction of the interstellar gas. Subject headings: molecules, interstellar