The CO(2) adsorption characteristics of prototypical poly(ethyleneimine)/silica composite adsorbents can be drastically enhanced by altering the acid/base properties of the oxide support via incorporation of Zr into the silica support. Introduction of an optimal amount of Zr resulted in a significant improvement in the CO(2) capacity and amine efficiency under dilute (simulated flue gas) and ultradilute (simulated ambient air) conditions. Adsorption experiments combined with detailed characterization by thermogravimetric analysis, temperature-programmed desorption, and in situ FT-IR spectroscopy clearly demonstrate a stabilizing effect of amphoteric Zr sites that enhances the adsorbent capacity, regenerability, and stability over continued recycling. It is suggested that the important role of the surface properties of the oxide support in these polymer/oxide composite adsorbents has been largely overlooked and that the properties may be even further enhanced in the future by tuning the acid/base properties of the support.