Reaction rates for the conversion of ice nanocrystals within 3-D arrays to mono-, tetra- and hexahydrates of HCl have been determined for different HCl(g) exposure levels and reaction temperatures. The extent of reaction was monitored and the hydrate product identified, using transmission FT-IR spectroscopy. The direct method of observation provides evidence that HCl diffusion through the hydrate encrusting the ice particles is often the rate-limiting factor in the conversion of ice to a hydrate phase. Values for the HCl diffusion coefficient for the mono- and tetrahydrate are reported. By comparison, diffusion of HCl in ice is considered to be a nonfactor in most ice-HCl interactions.