The kinetics of hydrolysis of cis-[CoX(bzmH)(en₂)]²⁺(X = Cl or Br; bzmH = benzimidazole; en = ethylenediamine) cations have been investigated in perchlorate medium of I= 0.3 mol dm^–3 and at 20–60 °C. In the range pH 6.8–8.5 and 20–40 °C the rate law for aquation takes the form –dln[Co^III]/dt=(k₁, +k₂K_NH[H⁺]^–1)/(1 +K_NH[H⁺]^–1) where k₁ and k₂ are the aquation rate constants of [CoX(bzmH)(en)₂]²⁺ and [CoX(bzm)(en)₂]⁺ respectively and K_NH is the acid-dissociation constant of the co-ordinated benzimidazole. The pK_NH of benzimidazole in [CoCl(bzmH)(en)₂]²⁺ is 8.6 at 25 °C and I= 0.3 mol dm^–3 from pH-titration and spectrophotometric measurements. Co-ordinated benzimidazole is 10⁴ times stronger as an acid than free benzimidazole. The labilrzing action of benzimidazole on the Co–X bond is stronger than that of imidazole in the k₁ path. This effect is, however, reversed in the k₂ path [i.e. k₂(im) > k₂(bzm)]. The imido-base [CoX(bzm)(en)₂]⁺ labilizes the Co–X bond ca. 700 times stronger than its conjugate-acid analogue.