A kinetic study of CO 2 hydration was carried out using the water-soluble zinc model complex with water-soluble nitrilotris(2-benzimidazolylmethyl-6-sulfonate) L1S, [L1SZn(OH 2 )] - , mimicking the active site of carbonic anhydrase, in the presence and absence of anion inhibitors NCS - and Cl - . The obtained rate constants k c a t for CO 2 hydration were 5.9x10 2 , 1. 7x10 3 , and 3.1x10 3 M - 1 s - 1 at 5, 10, and 15 o C, respectively: the k c a t =ca. 10 4 M - 1 s - 1 extrapolated towards 25 o C has been the largest among the reported k c a t using zinc model complexes for carbonic anhydrase. It was also revealed that NCS - , Cl - and acetazolamide play a role of inhibitors by the decrease of k c a t : 7x10 2 and 2x10 3 M - 1 s - 1 for NCS - and Cl - at 15 o C, respectively. The sequence of their magnitudes in k c a t is Cl - ~acetazolamide>NCS - , where the sequence Cl - >NCS - is confirmed for native carbonic anhydrase. The difference of k c a t or k o b s between NCS - and Cl - resulted from that between the stability constants K s t =2x10 3 for [L1SZn(NCS)] 2 - and 1x10 2 M - 1 for [L1SZnCl] 2 - in D 2 O: for water-insoluble tris(2-benzimidazolylmethyl)amine L1, K s t =1.8x10 4 for [L1Zn(NCS)] 2 - and 1.5x10 3 M - 1 for [L1ZnCl] 2 - in CD 3 CN/D 2 O (50% v/v). The crystal structure of anion-binding zinc model complexes [L1Zn(OH 2 )] 0 . 5 [L1ZnCl] 0 . 5 (ClO 4 ) 1 . 5 1 0 . 5 2 0 . 5 (ClO 4 ) 1 . 5 was revealed by X-ray crystallography. The geometry around Zn 2 + in 1 and 2 was tetrahedrally coordinated by three benzimidazolyl nitrogen atoms and one oxygen atom of H 2 O, or Cl - .