A regenerable high capacity carbon dioxide (CO2) sorbent is operated to remove substantially all of the CO2 present in either a dry, or a humid environment. The sorbent is an amine compound which is composed predominantly of secondary amine groups, and includes one or more functional nitrile groups. Primary and tertiary amine groups form a minor percent of the sorbent. The reaction product of tetraethylenepentamine (TEPA) and acrylonitrile (AN), which reaction product will be referred to hereinafter as “TEPAN” is a preferred sorbent. The addition of one or more nitrile functional groups to TEPA by reason of the reaction between AN and TEPA converts most of the primary amines in TEPA to secondary amines in TEPAN. We have discovered that the presence of predominantly secondary amines in the sorbent enhances the regenerable cyclic CO2 absorption capacity of the sorbent by more than two and one-half times, as compared to a TEPA sorbent bed; and the use of TEPAN as the sorbent bed also significantly reduces amine volatility to low levels which are immeasurable. The sorbent bed of this invention is also easily regenerable.