The structure of methylcobaloximes containing pyridine, (I), and 3-N-methylimidazole, (II), are reported together with a single-crystal i.r. study of the 3-N-methylimidazole derivative. Structural evidence for a hydroxocobaloxime with 3-N-methylimidazole, (III), is also reported. Crystals of (I) are triclinic, space group P, with cell parameters: a= 14.38(1), b= 10.02(1), c= 9.41(1)Å, α= 56.3(1), β= 127.3(1), γ= 106.6(1)°. Crystals of (II) and (III) are both monoclinic, space group P2₁/c, with cell parameters: (II)a= 9.25(1), b= 11.77(1), c= 19.80(1)Å, β= 124.4(1)°; and (III)a= 9.24(1), b= 11.55(1), c= 19.80(1)Å, β= 124.3(1)°. A total of 3 083 (I), 1 801 (II), and 1 739 (III) independent reflexions were used; the structures were solved from diffractometer data by the heavy-atom method and refined by least squares to final R factors of 0.064 (I), 0.051 (II), and 0.043 (III). Evidence of structural trans- and cis-effects is reported. Possible correlations between the structural features and chemical behaviour of cobaloximes and analogous quadridentate Schiff-base complexes are discussed.