The hydrogenation reactions of diphenylcarbene 1, fluorenylidene 2, and dibenzocycloheptadienylidene 3 were investigated in solid H2 and D2 matrices and in H2‐ and D2‐doped argon matrices at cryogenic temperatures. The reactivity of the carbenes towards H2 increases in the order 1<3<2. Whereas 1 is stable in solid H2, 2 and 3 react fast under the same conditions via quantum chemical tunneling. In D2 both 1 and 3 are stable, whereas 2 slowly reacts. The different reactivity of the three carbenes is rationalized in terms of differing carbene stabilization energies.