H/D isotopic exchange between H2O and D2O molecules was studied at the surface of ice films at 90-140 K by the technique of Cs+ reactive ion scattering. Ice films were deposited on a Ru(0001) substrate in different compositions of H2O and D2O and in various structures to study the kinetics of isotopic exchange. H/D exchange was very slow on an ice film at 95-100 K, even when H2O and D2O were uniformly mixed in the film. At 140 K, H/D exchange occurred in a time scale of several minutes on the uniform mixture film. Kinetic measurement gave the rate coefficient for the exchange reaction, k(140 K)=1.6(+/-0.3)x10(-19) cm(2) molecule(-1) s(-1) and k(100 K)less than or equal to5.7(+/-0.5)x10(-21) cm(2) molecule(-1) s(-1) and the Arrhenius activation energy, E(a)greater than or equal to9.8 kJ mol(-1). Addition of HCl on the film to provide excess protons greatly accelerated the isotopic exchange reaction such that it went to completion very quickly at the surface. The rapid reaction, however, was confined within the first bilayer (BL) of the surface and did not readily propagate to the underlying sublayer. The isotopic exchange in the vertical direction was almost completely blocked at 95 K, and it slowly occurred only to a depth of 3 BLs from the surface at 140 K. Thus, the proton transfer was highly directional. The lateral proton transfer at the surface was attributed to the increased mobility of protonic defects at the molecularly disordered and activated surface. The slow, vertical proton transfer was probably assisted by self-diffusion of water molecules. (C) 2004 American Institute of Physics.