Excitation transfer occurring as a result of an electron exchange mechanism. It requires an overlap of the wavefunctions of the energy donor and the energy acceptor. It is the dominant mechanism in triplet-triplet energy transfer. The transfer rate constant, \(k_{\mathrm{ET}}\), is given by: \[k_{\rm{ET}}\propto \frac{h}{2\ \pi }\ P^{2}\ J\ \rm{e}^{\frac{-2\ r}{L}}\] where \(r\) is the distance between donor (D) and acceptor (A), \(L\) and \(P\) are constants not easily related to experimentally determinable quantities, and \(J\) is the spectral overlap integral. For this mechanism the spin conservation rules are obeyed.