Cyclopropylhydroxycarbene was generated by high-vacuum flash pyrolysis of cyclopropylglyoxylic acid at 960 degrees C. The pyrolysis products were matrix-isolated in solid Ar at 11 K and characterized by means of IR spectroscopy. Upon photolysis, the carbene undergoes ring expansion, thereby paralleling the reactivity of other known cyclopropylcarbenes. The ring expansion product, cyclobut-1-en-1-ol, was characterized for the first time. Matrix-isolated cyclopropylhydroxycarbene undergoes [1,2]H-tunneling through a barrier of approximately 30 kcal . mol(-1), yielding cyclopropylcarboxaldehyde. The cyclopropyl moiety acts as a pi-donor and increases the half-life by almost a factor of 10 compared to parent hydroxymethylene, resulting in a temperature-independent half-life of tau = 17.8 h at both 11 and 20 K. Hence, cyclopropylhydroxycarbene is the first hydroxycarbene that differs from other members of its family by a significantly prolonged half-life. As expected, the O-deuterated analogue does not show tunneling. Our findings are rationalized by accurate CCSD(T)/cc-pVnZ (n = D, T)//M06-2X/6-311++G(d,p) computations. The half-life of cyclopropylhydroxycarbene was verified by tunneling computations employing the Wentzel-Kramers-Brillouin formalism. By comparison with other experimentally known hydroxycarbenes, we determine the electronic donor capabilities of the carbenes' substituents to be a dominant factor governing their half-lives.