Department of Chemistry, University of Bardwan, Burdwan-713 104 Manuscript received 27 July 1987, revised 9 December 1987, accepted 24 December 1987 The title reaction has been studied spectrophotometrically. The following rate law has been established in the pH range 4.64 — 5.3, \({{d[Cr(H_2O)(OH)(PyAcO)_2]} \over dt}=\) \(x = {{k_aK_E[Cr(H_2O)_5(OH)^{2+}]total[PyAcOH]} \over 1+K_E[PyAcOH]}\) where, KE is ion-pair equilibrium constant between the two reacting species and ka the rate constant for conversion of outersphere complex into innersphere complex. The reaction rate is found to be pH-dependent in the pH range 4.46 —5.3. The reaction rate is higher compared to the rate of isotopic water exchange process of the same substrate complex. Activation parameters have been calculated. A mechanism involving outersphere associative equilibrium between the two reacting species followed by associative interchange (Ia) for outersphere complex to product has been suggested.