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Time resolved electron paramagnetic resonance spectra and the decay kinetics of spin correlated radical pair (SCRP) polarization in an acyl-benzyl biradical were measured over a wide temperature range (180-274 K). The major mechanism of intersystem crossing in this biradical is the spin rotation induced relaxation of the acyl moiety, which is associated with the rotation of the carbonyl group about the neighbouring CC bond axis. This relaxation determines the decay rate of the polarization. The relaxation time is largely viscosity independent; it changes by a factor of less than two going from room temperature (60 ns) to 180 K (110 ns) in 2-propanol. [ABSTRACT FROM AUTHOR]