Viscosity and conductometric studies have been made for the solutions of carbonatopentaamine Co(Ill) nitrate, aquopentaamine Co(Ill) nitrate, oxalatopentaamine Co(Ill) nitrate, malonatopentaamine Co(Ill) nitrate and succinatopentaamine Co(Ill) nitrate in water, water+mannitol (5wt%) and water + sorbitol (5 wtV~) at 30°C. Association constants for the complexes in these solvents have also been evaluated. The results are discussed in terms of ion-ion and ion-solvent interactions, and of structural effects on the solvent in solution.Keywords. Octahedral Co(Ill) complexes; association constant; ion-ion and ionsolvent interactions.