The decomposition of 2,2-azobis(isobutyronitrile) (AIBN) at room temperature was studied in emulsion gels, with and without silica, using UV/visible spectroscopy. The emulsion gels consisted of toluene, AIBN, an aqueous solution of a surfactant (either hexadecyltrimethylammonium bromide, CTAB, or sodium dodecyl sulfate, SDS), and, in some cases, fumed silica. The AIBN compositions were determined in the opaque emulsion gels by converting them to transparent microemulsions. The decomposition rate constants for AIBN were determined to be 3.7 x 10(-8) s(-1) (0.6 x 10(-8) s(-1)) for the emulsion gels and 10.2 x 10(-8) s(-1) (1.3 x 10(-8) s(-1)) for the silica-containing emulsion gels. These rate constants were significantly greater than that in toluene solution, which, by our technique, is below our measurement threshold (effectively 0). (c) 2006 Elsevier Inc. All rights reserved.