A Kharash-type Grignard cross-coupling reaction of phenyl-d₅-magnesium bromide and an appropriate diiodo compound in the presence of bis (acetylacetonato) nickel (II) was successfully employed for the syntheses of twelve new decadeuterated polyphenyls, each of which has phenyl-d₅ rings at both termini. On the basis of the nuclear magnetic resonance spectra of the deuterated and normal polyphenyls, close correlations between the apparent signal patterns of phenylene groups and the structures of the adjacent groups became apparent. The mass spectral studies revealed that the deuterated polyphenyls containing o-linkage (s) were less stable to electron bombardment than those containing no o-linkage. Inspection of the ultraviolet spectra of the deuterated polyphenyls showed that the isotope effect in their K-bands was hardly detectable.