Aluminum atoms were reacted with 1,2butylene sulfide under matrix isolation conditions in a metal atom reactor known as a "rotating cryostat". The resulting paramagnetic products were characterized using electron paramagnetic resonance (EPR) spectroscopy. The effect of isotopic substitution and temperature on the spectral lines of the paramagnetic products helped in the identification of the species formed in the reaction. The results suggest that Al atoms favor insertion into the C 1 −S bond, forming primarily 4-ethyl-2-aluminathietane. Support for the spectral assignments was obtained by comparing the experimental magnetic parameters with those calculated using density functional theory. As the reaction mixture was annealed in the cavity of the EPR spectrometer, the cis-methallyl radical was detected. Aluminum is thought to mediate the desulfurization of 1,2-butylene sulfide.