The zero-field-splitting D parameter of 11 meta,para-disubstituted 1,3-diarylcyclopentane-1,3-diyl triplet diradicals has been determined in a 2-MTHF glass matrix at 77 K by EPR spectroscopy. The experimental Delta D-exp value, conveniently defined as D-H - D-X, is a sensitive function of the delocalizing property of the X substituent on the phenyl ring. In contrast to the previously observed additivity for a large variety of para and meta substituents , such a simple relation no longer upholds for meta,para disubstitution. For the latter, the higher degree of spin delocalization is accounted for in terms of spin polarization and electronic field effects. For the first time also the increased spin density at the cumyl position caused by meta substituents has been satisfactorily explained by means of spin polarization. The spin density distribution in the cumyl radicals, computed by density functional theory (B3LYP/6-31g*), substantiate these experimental findings.