The isotopic effects of sublimating ice is poorly understood and disagreement from diverging results from studies spans decades. The core question is whether sublimation occurs layer-by-layer with no fractionation or whether diffusion within the ice and vapor-ice exchange generate fractionation. Here, small ice spheres were suspended in an unsaturated atmosphere and a Rayleigh distillation model was used to estimate fractionation of the spheres. A small, yet statistically significant and repeatable, fractionation (103lna18O of ~ -0.6‰ (ɑ = 0.999) and 103lna2H -3 to -6‰ (ɑ = 0.994 to 0.997 ) was observed, smaller than predicted for equilibrium fractionation at this temperature and humidity. Assuming a modest porosity of 0.0005%, porosity could sufficiently increase diffusivity to explain the observed fractionation. The results help reconcile how sublimation varies between experimental and observational studies where uncontrolled porosity varies substantially across a continuum from porous firn layers to low porosity ice deep in glaciers.