The determination of the product ratios of the reactions of 3 to investigate ICS is based on identifying the characteristic IR bands (Figure ) and the corresponding, red-shifted bands of the isotopolog. Specifically, the strongest vibrational signal for each product was identified and integrated. To accurately compare the isotopolog ratios, a correction factor was applied. This factor was derived from the computed intensity ratios of the vibrational signals in the deuterated and parent molecules, allowing for direct comparison between the undeuterated and deuterated peaks. The experimental intensities were then normalized using the correction factor (Tables ), ensuring that the observed data accurately reflect the underlying reaction dynamics. The intensities obtained for the H-and D-isotopologs were subsequently used to calculate the selectivity between the competing reactions (Tables ). As evident from the IR spectrum after HVFP, conducted three times for both H-/D-isotopologs (Figures and), the product distribution shows a ratio of 2:1 favoring 4 (Figure ). Substituting hydrogen with deuterium at the α-position of 13, leading to intermediate carbene d2-3, changed the product ratio to 20:1, favoring d2-4, which was also qualitatively predicted computationally (Table ). The markedly different product ratios highlight the significant impact of deuterium substitution, providing strong evidence for ICS. In conclusion, we photochemically generated 1 and 2 from their diazirine precursors, with 2 being only the second matrix-isolated aliphatic carbene. Carbene 1 underwent a QMT reaction as computationally predicted, with a half-life of 6 h, while 2 remained stable under cryogenic conditions but rapidly rearranged to 9 upon 627 nm irradiation. Much more reactive carbene 3 was generated in situ and a-deuterium substitution of the diazirene precursor significantly altered the ratio of the [1,2]H-shift vs. C-H-insertion products, thereby providing the first experimental evidence of isotopecontrolled selectivity.