4-(3,4-Dimethyl-phenyl)-1(2H)-phthalazinone was used as key starting material for synthesis a new series of 1,4- disubstituted phthalazines, 2,4-disubstituted phthalazinones, and fused 1,2,4-triazolo-phthalazines depending on the principles of lactam-lactim dynamic equilibrium phenomena. The structure of the prepared compounds are elucidated using physical and spectral data like, FT-IR, 1HNMR, and mass spectroscopy.