The sensitivity of the half life of Eu(II) luminescence to changes in electrolyte concentration has been studied. The values of n, the hydration number, were calculated and found to increase for HClO4, LiClO4 and NaClO4 solutions and to decrease for NaCl and LiNO3 Solutions as the concentrations were increased. In HCl solution, n increased with concentration to 5 M, then decreased. These observations are discussed in terms of models of outer vs. inner sphere complexation and of increased interaction of the primary hydration sphere with the Eu(II) cation.