New and complementary viscosity data for aqueous N-methyldiethanolamine-piperazine (MDEA-PZ) solutions with and without CO2 are presented in this paper for different temperatures and mass fractions. The first part of this work represents viscosity of pure and aqueous MDEA to validate the measurement system. The second part represents viscosity of unloaded aqueous MDEA-PZ solutions with total mass fractions of MDEA ranging from 0.20 to 0.40 and PZ mass fractions at 0.05 and 0.1. Two different techniques for representing viscosity of the unloaded systems are discussed. The third part of this paper reports viscosities of CO2-loaded aqueous MDEA-PZ at different CO2 loadings. These data were correlated using a modified Setschenow equation.