For the first time in an application to nuclear astrophysics, a process induced by the unstable 5 He = ( 4 He-n) nucleus, the 3 He+ 5 He → 2 α reaction, has been studied through the Trojan Horse Method (THM). For that purpose, the quasi-free (QF) contribution of the 9 Be( 3 He, α α ) 4 He reaction was selected at E 3 He = 4  MeV incident energy. The reaction was studied in a kinematically complete experiment following a recent publication (Spitaleri et al. in Eur Phys J A 56:18, 2020), where for the quasi free contribution the momentum distribution between α and 5 He particle cluster in the 9 Be nucleus in the ground state have been extracted. The angular distribution of the QF 3 He+ 5 He → 2 α reaction was measured at θ cm = 78 ∘ –115 ∘ . The energy dependence of the differential cross section of the 3 He+ 5 He → 2 α virtual reaction was extracted in the energy range E cm = 0–650 keV. The total cross section obtained from the Trojan-horse method was normalized to absolute cross sections from a theoretical calculation in the energy range E cm =300–620 keV.