Cavity ring-down absorption spectroscopy (CRDS) is employed to investigate one-photon dissociation of (COCl)(2) at 248 nm obtaining a primary Cl-2 elimination channel. A ratio of vibrational population is estimated to be 1:(0.12 +/- 0.03):(0.011 +/- 0.003) for the v = 0, 1, and 2 levels. The quantum yield of Cl-2 molecular channel is obtained to be 0.8 +/- 0.4 initiated from the (X) over tilde (1)A(g) ground state surface (COCl)(2) via internal conversion. The obtained total quantum yield is attributed to both primary ((COCl)(2) + hv -> 2CO + Cl-2) and secondary reactions (dominated by Cl + COCl -> Cl-2 + CO). The former is estimated to share a yield of >0.14, while the latter contributes up to 0.66. The photodissociation pathway to the molecular products is calculated to proceed via a four-center transition state (TS) from which Cl-2 is eliminated synchronously. Installation of the mirrors with reflectivity of 99.995% in the CRDS apparatus prolongs the ring-down time to 70 mu s, thus allowing for the contribution from 17% up to 66% of the total Cl-2 yield from secondary reaction depending on the reaction temperature. Despite uncertainty in determining the product yield, the primary Cl-2 dissociation channel eliminated from (COCl)(2) is observed for the first time.