The stability constants of the 1:1 complexes formed between Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+ Zn2+, or Cd2+ (=M2+) and 1-methyl-4-aminobenzimidazole (MABI) or 1,4-dimethylbenzimidazole (DMBI) were determined by potentiometric pH titrations in aqueous solution (25 degreesC; I = 0.5 M, NaNO3). Some of the stability constants were also measured by UV spectrophotometry. The acidity constants of the species H-2(MABI)(2+) and H(DMBI)(+) were determined by the same methods, some twice. Comparison of the stability constants of die M(MABI)(2+) and M(DMBI)(2+) complexes with those calculated frorn logK(ML)(M) versus pK(HL)(H) straight-line plots, which were established previously for sterically unhindered benzimidazole-type ligands (=L), reveals that the stabilities of the M(MABI)(2+) and M(DMBI)(2+) complexes are significantly reduced due to steric effects of the C4 substituents on metal ion binding at N3. This effect is more pronounced in the M(DMBI)(2+) complexes. Considering the steric equivalence of methyl and (noncoordinating) amino groups (as they occur in adenines), it is concluded that the same extent of steric inhibition by the (C6)NH2 group is to be expected on metal ion binding at N7 with adenine derivatives. The basicity of the amino group in MABI is significantly higher than in its corresponding adenine derivative. Indeed, it is concluded that in the M(MABI)(2+) complexes chelate formation involving the amino group occurs to some extent. The formation degrees of these "closed" species are calculated; they vary for the complexes of Mn2+, Co2+, Ni2+, Cu2+, Zn2+, or Cd2+ between about 50 and 90%. The stability of the M(MABI)(2+) and M(DMBI)(2+) complexes with the alkaline earth ions is very low but unaffected by the C4 substituent; this probably indicates that in these instances outersphere complexes (with a water molecule between N3 and the metal ion) are formed.