Delivery of NO to specific targets is important in fundamental studies and therapeutic applications. Various methods have been reported for delivery of NO in vivo and in vitro; however, there are few examples of systems that reversibly bind NO. Reported herein is the development of a new polymer (P-1[Co-II]) that reversibly binds NO. P-1[Co-II] has a significantly higher affinity for NO compared to O-2, CO2, and CO. The polymer is synthesized by template copolymerization methods and consists of a porous methacrylate network, containing immobilized four-coordinate Co-II sites. Binding of NO causes an immediate color change, indicating coordination of NO to the site-isolated Co-II centers. The formation of P-1[Co(NO)] has been confirmed by EPR, electronic absorbance, and X-ray absorption spectroscopies. Electronic and X-ray absorbance results for P-1[Co-II] and P-1[Co(NO)] show that the coordination geometry of the immobilized cobalt complexes are similar to those of their monomeric analogues and that NO binds directly to the cobalt centers. EPR spectra show that the binding of NO to P-1[Co-II] is reversible in the solid state; the axial EPR signal associated with the four-coordinate Co-II sites in P-1[Co-II] is quenched upon NO binding. At room temperature and atmospheric pressure, 40% conversion of P-1[Co(NO)] to P-1[Co-II] is achieved in 14 days; under vacuum at 120 degreesC this conversion is complete in similar to1 h. The binding of NO to P-1[Co-II] is also observed when the polymer is suspended in liquids, including water.