An apparatus developed to measure absorption kinetics of acid gases by amine solutions is described as well as the experimental procedures dealing with either the initial kinetics or kinetics for loaded solutions. Data obtained for H2S absorption into a 50 mass % methyldiethanolamine (MDEA) aqueous solution with various initial H2S loadings are reported for the following conditions : H2S loadings from 0 to 0.44 mol H2S per mol MDEA and two temperatures, 296 and 343 K. The H2S experimental absorption rates have been used together with a model, based on mass transfer and involving an instantaneous reversible reaction, to determine the MDEA diffusion coefficient.