A new ice-catalyzed chemical reaction is reported, namely the addition of Cl-2 to the C-C double bond of propene (C3H6) to form 1,2-dichloropropane. The reaction, which was investigated with temperature-programmed desorption mass spectrometry (TPD), was carried out on ultrathin (10-100 monolayer thick) films of ice deposited on single-crystal metal substrates under ultrahigh vacuum. The Cl-2-addition product was identified as 1,2-dichloropropane on the basis of its fragmentation pattern in the mass spectrometer. 1,2-Dichloropropane formation occurs below 150 K, and no chlorohydrin (e.g., 2-chloro-1-propanol) evolution is ever observed. The reaction of coadsorbed propene and chlorine is very different from what occurs in aqueous solutions, where chlorohydrin formation occurs readily. Possible mechanisms of the ice-surface-catalyzed reaction are discussed.