A process for preparing 1-chloro-1,1,3,3,3-pentafluoropropane, CF3CH2CF2Cl, comprising contacting in a reaction zone in the substantial absence of oxygen, reactants comprising chlorine and 1,1,1,3,3-pentafluoropropane, CF3CH2CHF2 (also referred to as HFC-245fa), and subjecting the reactants to actinic radiation, such as UV light at about 2,000 to 4,000 Angstroms, wherein: (1) inert gas is present at a concentration equal to or less than about 5 wt.% of the total weight of reactants; (2) the molar ratio of chlorine to CF3CH2CHF2 is from about 0.2:1 to about 1.5:1; and (3) the concentration of chlorinated product produced having greater than one chlorine present in the molecule is less than or equal to about 10 wt.%.