Solutions of the amines N-methyldiethanolamine (MDEA) and piperazine (PZ) are commonly used for CO2 removal in industrial applications. This is mainly due to the low heat of reaction of MDEA, reducing the heat required for regeneration, and the high reaction rate of PZ, which reduces the size of the absorption tower. However, the effect of PZ on the heat of absorption in a mixture of these two solutions has not previously been investigated. This paper presents experimental data on the heat developed during the absorption of CO2 in aqueous solutions of MDEA and PZ, and mixtures thereof, at low loading and temperatures from 35 °C to 65 °C. The results indicate that PZ affects the amount of heat developed to a higher degree than previously thought, resulting in a significant increase compared to pure MDEA solutions. This effect is more pronounced at low carbon dioxide loadings. The impact of the temperature on the heat of absorption is weak for MDEA, as has been found in previous studies. The present study also demonstrated a weak temperature dependence for PZ.