Alpha-particle emission associated with the β decay of B12 has been detected and the energy spectrum investigated. These studies show that (1.3+/-0.4)% of all decays of B12 lead to the second excited state of C12 and that this state breaks up predominantly into three alpha particles with one alpha particle and the ground state of Be8 as an intermediate stage in the process. The most probable spin and parity assignments for the state appear to be J=0+, and analysis of the alpha-spectrum yields Q(C12*-Be8-He4)=278+/-4 kev, corresponding to an excitation energy in C12 of 7.653+/-0.008 Mev. A new determination of the disintegration energy of Be8 yields Q(Be8-2He4)=93.7+/-0.9 kev and hence Q(C12*-3He4)=372+/-4 kev. It is concluded, from the general principle of reversibility of nuclear reactions, that the second excited state of C12 as predicted by Hoyle is of a suitable character to act as a stellar thermal resonance in the Salpeter process, 2He4⇌Be8 Be8(α, γ)C12 under conditions expected in red giant stars.