A new reaction is reported on the ice surface, namely the conversion of adsorbed 2-methyl-2-propanol [(CH3)(3)COH] to adsorbed 2-chloro-2-methylpropane [(CH3)(3)CCl] in the presence of absorbed HCl. The reaction was studied using temperature-programmed desorption mass spectrometry on ultrathin ice films under ultrahigh vacuum. Formation of (CH3)(3)CCl occurs at two temperatures: at or below 155 K and at 185 K. The lower-temperature process occurs at the surface of the film, but the origin of product at 185 K is unclear. The yield of the surface reaction product is low, approximately 0.04 monolayers. Importantly, adsorbed HCl is unreactive toward a;methyl-a-propanol; reactive chlorine is derived from a dissociatively ionized state in the very near surface region of the film. The lower-temperature pathway for chloride substitution is observed only at the surface of an amorphous film, an observation that may have implications regarding the nature of surface reaction sites. Possible mechanisms of the surface reaction are discussed.