The diffusion of HDO into ultrathin single-crystal (H2O)-O-16 ice multilayers was investigated using a novel combination of laser-induced thermal desorption (LITD) probing and isothermal desorption depth-profiling. The single-crystal hexagonal ice multilayers were grown epitaxially on a Ru(001) metal substrate, and the diffusion coefficients were measured perpendicular to the basal (0001) facet. The measured HDO diffusion coefficients ranged from D = (2.2 +/- 0.3) x 10(-16) to D = (3.9 +/- 0.4) x 10(-14) cm(2)/s over the temperature range 153-170 K. Arrhenius analysis of the temperature-dependent diffusion coefficients yielded a diffusion activation energy of E-A = 17.0 +/- 1.0 kcal/mol and a preexponential factor of D-0 = (4.2 +/- 0.8) x 10(8) cm(2)/s. The similarity of the diffusion coefficients for HDO and (H2O)-O-18 indicates that H/D exchange does not contribute significantly to HDO diffusion in ice. The agreement between the diffusion kinetics for HDO and (H2O)-O-18 argues that the HDO diffusion occurs via a molecular transport mechanism. The large diffusion preexponentials for both HDO and (H2O)-O-18 diffusion into the ultrathin ice multilayers also suggest that built transport properties in ice may be perturbed by close proximity to the ice surface.