Formyl radical has been detected in zeolite NaY at room temperature by step-scan FT-infrared spectroscopy upon photodissociation of glycolaldehyde or acetaldehyde precursor. Identification was made by the C=O stretch absorption at 1847 cm(-1) and a C-13 isotope shift of 40 cm(-1). The decay shows biphasic kinetics independent of the precursor used. In the case of glycolaldehyde, the final products recorded by static FT-IR spectroscopy can be explained by the exclusive reaction of HCO and CH2OH radicals. This allowed us to assign the initial decay with a lifetime of 24 +/- 3 us to geminate reactive encounters of HCO and CH2OH radicals. The subsequent long tail extending to 500 mus is well described by a second-order rate law consistent with nongeminate reaction of the radicals. This is the first direct kinetic observation of geminate and nongeminate radical reactions in a zeolite.