Elusive tert-butylhydroxycarbene was generated in the gas phase via high-vacuum flash pyrolysis of tert-butylglyoxylic acid at 960 °C. The pyrolysis products were subsequently matrix isolated in solid Ar at 11 K and characterized by means of IR spectroscopy. While still being exposed to the harsh pyrolysis conditions, the hydroxycarbene undergoes CH-insertion to dimethylcyclopropanol, as well as a CC-insertion to novel methylbutenol, with activation barriers of 23.8 and 31.0 kcal mol−1, respectively. Once embedded in the cold Ar matrix, the carbene transforms to its isomer pivaldehyde not only by photolysis, but it also cuts through the barrier of 27.3 kcal mol−1 by quantum mechanical tunneling. The temperature independent half-life is measured as 1.7 h; the tunneling pathway was entirely blocked upon O-deuteration. The experimental half-life of tert-butylhydroxycarbene was verified by tunneling computations applying the Wentzel–Kramers–Brillouin formalism on the minimum energy path evaluated at the computationally feasible M06-2X/6-311++G(d,p) level of theory. Our experimental findings are supported by relative energy computations at the CCSD(T)/cc-pVDZ level of theory.