We examine the chemical reaction of isodiiodomethane (CH2I-I) with cyclohexene using time-resolved resonance Raman spectroscopy. Our results indicate that CH2I-I reacts with cyclohexene to produce an iodine molecule leaving group on the 5-10 ns time scale which then almost immediately forms a I-2:cyclohexene complex. This in conjunction with previous results from photochemistry experiments and recent density functional theory calculations indicates that isodiiodomethane is the methylene transfer agent mainly responsible for cyclopropanation of olefins when using ultraviolet photolysis of diiodomethane. We present a mechanism for photocyclopropanation that is consistent with both experimental and theoretical characterization of reaction intermediates and products formed after ultraviolet photolysis of diiodomethane in a condensed phase environment. We briefly discuss the differences and similarities in the behavior of the isodiiodomethane and the Simmons-Smith carbenoids toward cyclopropanation reactions with olefins.