AbstractIn reactions of 1‐phenyl‐1,4‐dihydro‐3‐(2H)benzoisoquinolinones with benzaldehyde in the presence of a strong base, the final result can be ring substitution with aromatization or only aromatization of the starting isoquinolinone, depending on the position of the ring junction ([f] or [h]), in the 5‐methyl derivative the latter process becomes predominating. In the system sodium hydride‐dimethylformamide, containing no aromatic aldehyde, methylation at site 4 occurs. Using compounds labelled with deuterium, it has been established that the 4‐benzyl or 4‐methyl derivatives with aromatic structures are formed through hydri