Absorption rates of gaseous CO2 into aqueous blends of N-methyldiethanolamine (MDEA) and diethanolamine (DEA) and of gaseous H2S into aqueous MDEA were measured in a quiescent, inverted-tube diffusiometer by monitoring the rate of pressure drop. A numerical model for absorption, diffusion, and reaction of CO2 and H2S in blends of MDEA, DEA, and water was developed. The model was used to regress diffusion coefficients of bicarbonate, carbamate, and MDEAH(2)CO(3) for the case of CO2 absorption and of bisulfide ion for the case of H2S absorption from measured absorption rates. CO2 absorption rates and diffusion coefficients of bicarbonate, carbamate, and MDEAH(2)CO(3) were obtained at 298.2 K and 318.2 K in aqueous solutions containing 50 mass % total amine at DEA:MDEA mole ratios of 1:20, 1:4, 1:3, and 2:3. H2S absorption rates and diffusion coefficients of bisulfide ion were obtained at 298.2 K and 318.2 K in aqueous solutions containing 20, 35, and 50 mass % MDEA.