The reaction of R(Ba1.5Sr0.5)Cu3Oy (RS123, R = La, Nd, Sm, Eu, Gd, Dy, Ho) powder with water is studied quantitatively. The initial reaction rate law is R-0 = k[S](a)(0)[H3O+](b)(0) where [S](0) is the initial surface area of the RS123 superconducting powder. For the entire RS123 series, a is close to 1, indicating that the reaction rate is linearly dependent on the surface area of the powder. However, b is variable for the whole series. For R = La, Nd, Sm, Eu, Gd, Dy, and Ho, b equals to 0.75, 0.70, 0.62, 0.56, 0.51, 0.56, and 0.60, respectively. It is found that the stability of RS123 in water is correlated with the oxygen stoichiometry of RS123. The stability decreases with increasing the oxygen stoichiometry. (C) 2003 Elsevier B.V. All rights reserved.