The arylidene-isoxazolone compounds, 3-phenyl-4-(2,4,6-trimethylbenzylidene)isoxazol-5(4H)-one, 3a, and 3-phenyl-4-(2,4,6-trimethoxybenzylidene)isoxazol-5(4H)-one, 3b, were prepared and characterized by IR spectroscopy and single-crystal X-ray diffraction. The X-ray analysis revealed that at the solid state the two molecules adopt different configuration at the exocyclic C=C bond, Z for 3a and E for 3b, respectively. By using the density functional theory (DFT) method with 6-31+G(d,p) basis set, the molecular geometry, vibrational frequencies, atomic orbitals and potential electrostatic maps were calculated and compared with the experimental data. The calculated results showed that the optimized geometry can well reproduce the crystal structure. Calculations were performed about the thermodynamic energy related to both the configurational isomer models of 3a,b in the ground state. In addition, a study aiming to understand the effect of the it-donor methoxy substituents on the proton affinity of the nitrogen atom was carried out.