Densities and viscosities of aqueous solutions containing both piperazine (PZ) and N-methyldiethanolamine (MDEA) have been determined at a temperature range from (293.15 to 323.15) K. The concentrations of MDEA have been kept constant at (1, 2, 3, and 4) mol . dm(-3) with the concentration of PZ varying from (0 to 1) mol . dm(-3). Liquid diffusivities of aqueous PZ solutions have been determined using the Taylor dispersion 3 technique over a temperature range from (293.15 to 368.15) K and concentrations from (0 to 1.5) mol . dm(-3). infinite dilution, the method proposed by Othmer & Thakar can be used to estimate the liquid diffusivities, and a modified Stokes-Einstein relation can be used to predict the diffusivities at higher concentrations. Liquid diffusivities of both PZ and MDEA have been determined in aqueous solutions containing 4.0 mol . dm(-3) MDEA and (0, 0.5, and 1.0) mol . dm(-3) pZ at a temperature range from (298.15 to 368.15) K. The liquid diffusivities can be estimated by a modified Stokes-Einstein relation.