Formulation of efficient absorbing solutions for carbon dioxide capture is essential to reduce the operation costs of industrial gas treatment plants. Recently, mixed amine systems, typically composed of a tertiary and a primary or secondary amine, have been a growing interest in academic and industrial research. In particular, the so-called activated methyldiethanolamine (aMDEA), which contains small amounts of piperazine (PZ), has become one of the most promising new candidates. This work intends to perform a comparison between the performance of single amines on the one hand and their blends on the other hand with the aim of elucidating how PZ and MDEA can interact in the latter situation. In the first part, aqueous solutions made up of a pure reagent (either MDEA or PZ) were studied. Rates of CO2 transfer from a gas phase to unloaded and loaded solutions, up to nearly saturation points, were experimentally investigated for a wide range of operating conditions using a membrane contactor. Observed CO2 transfer rates were reproduced with a simple yet effective model that captured the main physical and chemical aspects.