Density functional UB3LYP calculations with the broken symmetry approach and spin-projection method on m-xylylene diradical and its derivatives have been performed to investigate the dependence of the spin-coupling constant through m-phenylene on the number of phenyl substituents on the two radical sites. The results show that the coupling constant (or the singlet–triplet gap) steadily decreases on increasing the number of phenyl substituents on the two radical sites. This trend is ascribed to more spin delocalization into the phenyl substituents and the larger twist angle of the phenyl substituents out of the m-phenylene plane when the degree of phenyl substitution is increased. For Schlenk's hydrocarbon, the coupling constant J is 1.52kcal/mol, only one quarter of that in the parent m-xylylene.