A set of one-bond CC coupling constants has been determined for mono-and disubstituted benzenes. Large 'J(CC) values have been found within the benzene rings bearing highly electronegative substituents such as halogens, methoxy and nitro groups and small values for those with electropositive subs&en&. The total range of 'J(CC) couplings observed in our work is larger than 5OHz. These large variations of CC spin-spin couplings are interpreted in terms of Fermi-contact contributions and the redistribution of s-electrons within a CC bond under influence of substituents. Contrary to some previous findings the data obtained in the present work indicate that the relationship between 'J(CC) and the substituent electronegativity is not linear. EXPERIMENTAL Solvents All solvents used in the present work, i.e. tetrahydrofuran, diethyl ether and benzene-b, were carefully dried using either lithium aluminum hydride (THF sad DEE) or sodium (GD6) and kept under argon atmosphere. Compounds Most of the compounds studied in the present work have been commercially available and they were used in measurements either without further purification or if necessary distilled prior to use. Phenyllithium was prepared from bromobenzene and lithium according to the procedure described in Ref. [6]. Arylmagnesium halides were obtained from the corresponding aryl halides and * Preliminary results were presented during the EUCMOS XXI Conference, Vienna, 23-28 August 1992.