Abstract 1. The character of the dπ- pπ bond formed between the phenyl group and the elements of Group IV/1 has been examined. On the basis of the ultraviolet spectra and dipole moments of the trimethylphenyl-E and trimethylbenzyl-E (where E represents carbon, silicon, germanium and tin) compounds synthesized by us, it has been established that the heteroatoms—except carbon— being directly bonded to the aromatic ring, become conjugatively connected with the aromatic system. 2. The bathochromic effect appearing in the ultraviolet spectrum of trimethylphenylsilane was proved by means of quantum-chemical calculations. The partial charge shares and the bond orders of the atoms in the molecule were computed. 3. It was proved by a variation method that the ultraviolet spectrum of the trimethylphenyl compounds must show a hypsochromic shift from silicon to tin. 4. In the series of the trimethylbenzyl-E compounds (except the carbon analogue) a considerable bathochromic shift is caused by the hyperconjugation effect of the CH 2-group, since the hyperconjugation appears in both direction (both towards the heteroatom and the phenyl group). 5. On the basis of the experimental data, the value of the electronegativity in Group IV/1 decreases in the following sequence: carbon, silicon, germanium, tin.