Abstract We have studied the effects of nine new phenolic compounds: 4-(4-hydroxy-3-methoxyphenyl)-but-3-en-2-one 4, 4-(4-hydroxy-3-methoxyphenyl)-but-3-en-2-one 5, 4-hydroxy-3,5-dimethoxycinnamyl alcohol or sinapyl alcohol 6, 4-hydroxy-3,5-dimethoxy- N, N-dimethylcinnamamide 7, N-ethyl-4-hydroxy-3,5-dimethoxycinnamamide 8, 1-(4-hydroxy-3,5-dimethoxyphenyl)-3-(4-hydroxy-3,5-dimethoxyphenyl)-prop-2-en-1-one 11, 1-(4-hydroxyphenyl)-3-(4-hydroxyphenyl)-prop-2-en-1-one 12, 4-(4-hydroxy-3,5-dimethoxyphenyl)-butan-2-one 13, and 1-(4-hydroxy-3,5-dimethoxyphenyl)-3-(4-hydroxyphenyl)-propan-1-one 14, on Agrobacterium virulence gene induction, on Agrobacterium-mediated gene transfer, and on both transient and stable transformation rates on Petunia and tobacco. We confirmed that virulence induction and transformation rates are increased by the use of phenolic vir inducers bearing an unsaturated lateral chain. However, we found that the presence of at least one ortho-methoxy group in the phenolic compounds is required to increase the vir gene induction. Moreover, we synthesized phenolic compounds 13 and 14 with a saturated lateral chain from the corresponding compounds 4-(4-hydroxy-3,5-dimethoxyphenyl)-but-3-en-2-one 2 and 1-(4-hydroxy-3,5-dimethoxyphenyl)-3-(4-hydroxyphenyl)-prop-2-en-1-one 9, in order to stop conjugation between hydroxyl and carbonyl groups in these molecules. These compounds showed a lower efficiency of both vir induction and gene transfer. Furthermore, we also synthesized two amides derivatives 7 and 8 from sinapinic acid. The nature of alkyl groups on nitrogen is essential to the vir induction and gene transfer, and we observed that N-dimethylamide compound 7 is less active than N-ethyl compound 8. This may be due to the difference of the electron-donating effect between methyl and ethyl groups. We present a model for the molecular mechanism of VirA activation by phenols.