Although frequently employed, heavy atom kinetic isotope effects (KIE) have not been reported for quantum mechanical tunneling reactions. Here we examine the secondary KIE through C-13-substitution of the carbene atom in methylhydrox-ycarbene (H3C-COH) in its [1,2]H-tunneling shift reaction to acetaldehyde (H3C-CHO). Our study employs matrix-isolation IR spectroscopy in various inert gases and quantum chemical computations. Depending on the choice of the matrix host gas, the KIE varies within a range of 1.0 in xenon to 1.4 in neon. A KIE of 1.1 was computed using the Wentzel Kramers Brillouin (WKB) CVT/SCT, and instanton approaches for the gas phase at the B3LYP/cc-pVTZ level of theory. Computations with explicit consideration of the noble gas environment indicate that the surrounding atoms influence the tunneling reaction barrier height and width. The tunneling half-lives computed with the WKB approach are in good agreement with the experimental results in the different noble gases.