Department of Chemistry, University of Kalyani, Kalyani-741 235 Manuscript received 18 February 1997, revised 15 July 1997, accepted 18 August 1997 Viscosity B-coefficients of a number of drugs (viz. hydrochlorides of cyproheptadine, chlorpromazine, ethambutol, amodiaquin and pheniramine maleate) in water have been determined at three different temperatures (293, 298 and 303 K). The single ion B+ values have been found to be high and positive but thed B+/dT values have been found to be negative. All drug cations can be regarded to be highly structure-making in character and highly solvated. This is due to hydrophobic hydrations of the drug molecules. The activation parameters have been found to be very high and positive. Naturally, the drug diffusion through the solution is extremely slow. The partial molar volumes of the drug molecules have been found to be high.