Aliphatic polyamines easily absorb acidic gases but unfortunately are not suitable for industrial application because their solutions are difficult; to regenerate. Molecular modification is of primary importance to reduce their basicity and to allow their use in acid gas recovery processes. Various modified polyamines were prepared by reacting tetraethylenepentamine (TEPA) with formaldehyde and formaldehyde/phenol in various molar ratios. The physical and chemical characterization of the synthesized products showed a general reduction of basicity; the modification of TEPA with formaldehyde and phenol gave products with high CO2 absorption capacity and easier regeneration; moreover, the heat of reaction during CO2 absorption was reduced. The selected compound had a TEPA/CH2O/C6H5OH ratio equal to 1/0.5/0.4. This new product showed higher cyclic capacity, higher absorption rate, and lower degradation compared with diethanolamine under the same experimental conditions. The product is stable, constant in composition, and suitable for industrial scale-up.