We have performed a survey of carbon dioxide capture in phosphonium-based ionic liquids and end-capped polyethylene glycol using DETA (DETA = diethylenetriamine) as a model absorbent. The carbon dioxide adduct of DETA is zwitterionic and forms readily in both ionic and polymeric media. Crystals of the adduct have been isolated, and its X-ray crystal structure is reported. The adduct is hygroscopic and decomposes with heating, with the onset of decomposition occurring at similar to 85 degrees C. Computational studies were performed examining possible structures for the adducts formed between DETA and CO2 in the gas phase. Also described is a study of the thermal stability of phosphonium ionic liquids and selected polymeric solvents under neutral and oxidizing conditions. Heat capacity measurements were also performed on a number of ionic liquids.