Iodine monochloride (ICl) elimination from one-photon dissociation of CH2ICl at 248 nm is monitored by cavity ring-down absorption spectroscopy (CRDS). The spectrum of ICl is acquired in the transition of B-3 Pi(0) <- X-1 Sigma(+) and is confirmed to result from a primary photodissociation, that is, CH2ICl + hv -> CH2 + ICl. The vibrational population ratio is determined with the aid of spectral simulation to be 1: (0.36 +/- 0.10):(0.11 +/- 0.05) for the vibrational levels v = 0, 1, and 2 in the ground electronic state, corresponding to a Boltzmann-like vibrational temperature of 535 +/- 69 K. The quantum yield of the ICl molecular channel for the reaction is obtained to be 0.052 +/- 0.026 using a relative method in which the scheme CH2Br2 -> CH2 + Br-2 is adopted as the reference reaction. The ICl product contributed by the secondary collisions is minimized such that its quantum yield obtained is not overestimated. With the aid of the CCSD(T)//B3LYP/MIDI! level of theory, the ICl elimination from CH2ICl is evaluated to follow three pathways via either (1) a three-center transition state or (2) two isomerization transition states. However, the three-center concerted mechanism is verified to be unfavorable.