The diffusion of moisture-related species into both tetragonal and cubic single crystals at temperatures characteristic of moisture-enhanced low-temperature degradation of tetragonal zirconia is reported using secondary ion mass spectrometry (SIMS). The crystals were immersed in D and O18 isotopically labeled water and in O18 gas environments over the temperature range of 100 degrees-220 degrees C and SIMS profiles of H, D, O18, and O18D determined. The oxygen diffusivities in both tetragonal and cubic crystals were the same, irrespective of whether the crystals were exposed to gas or water. Furthermore, the oxygen diffusivities were consistent with extrapolations of the oxygen diffusivities in cubic crystals reported at higher temperatures using the reported activation energy for oxygen vacancy diffusion. Both H and D diffuse into the tetragonal crystal when immersed in water but despite having a higher concentration of vacancies no diffusion of H or D was detectable in the cubic single crystals exposed under identical conditions. Quantification of the H and D diffusion into the tetragonal crystal proved unreliable due to charging effects even though the SIMS profiling was performed at liquid nitrogen temperatures. Nevertheless, it was concluded that D can diffuse in tetragonal zirconia at room temperatures over a period of several months. Finally, although a species of mass 20 amu, which is the same as the isotopic labeled OH (O18D) ion, was detected in the tetragonal single crystal zirconia exposed to water, it was concluded that this was a SIMS artifact and that moisture does not diffuse in zirconia as either an OH or H2O ion.