The temperature dependence of conversion of He(2\sp3S\sb1) metastable atoms to He\sb2(a\sp3Σ\sp+\sbu) metastable molecules in the three-body reaction$$\rm He(2\sp3S\sb1)+2He(1\sp1S\sb0) \to He\sb2(a\sp3\Sigma\sp+\sb{u})+He(1\sp1S\sb0)$$has been investigated over the temperature range 65K-700K. This reaction is thermally activated as a consequence of a long range repulsive barrier in the He(2\sp3S\sb1)-He(1\sp1S\sb0) interaction potential. The data reveal that there are two reaction channels with distinctly different activation energies. The temperature dependence of the measured rate coefficient δ(T) is accurately described by$$\rm \delta(T) = \lbrack 87.4\ T\ exp({-}750/T){+}4.1T\ exp({-}200/T)\rbrack \times 10\sp{-37}\ cm\sp{-6}sec\sp{-1}.$$The first activation energy, 750 ± 70K(63 ± 6meV), is equal to the known He(2\sp3S\sb1)-He(1\sp1S\sb0) repulsive barrier height. The second activation energy is 17 ± 2 meV.