INTRODUCTION Heterocycles are an important class of compounds not only due to their natural abundance but also because of their similarities with isoxazoline and isoxazolidine mainly due to their pharmacological activity 1. The isoxazoline derivatives have been reported to possess a variety of significant and diverse pharmacological activities such as antibacterial, anti-candida activity, antifungal, HIV-RT activity 2-6. Similarly, the isoxazolidine derivatives have been published to many pharmacological activities such as antibacterial, antiretroviral drug, hypoglycemic agents, anti-inflammatory, anti-tubercular, anticancer 7-12. They have been also reported earlier as corrosion inhibitors 13-14. Recently Yildirim et al. 15 have been investigated experimentally that isoxazoline and isoxazolidine derivatives showed inhibitive properties for iron and its alloys in 2 M HCl solutions 20 h at room temperature. In our previous study, we have performed quantum chemical calculations on acetamides and isoxazolidine derivatives, using the DFT and HF methods 16. The relationships between the structural parameters and corrosion inhibition of isoxazoline derivatives have not been studied yet. Therefore, the aim of this study is to present theoretical study on electronic and molecular structure of three isoxazoline 5-octylsulfanylmethyl-3-phenyl-4,5-dihydroisoxazol (OSPI), [1-(3-phenyl-4,5-dihydro-isoxazol-5-yl)nonan-2-ol (PINO), 9-(3-phenyl-4,5-dihydro-isoxazol-5-yl)nonanoic acid 2-[2-(methoxy-ethoxy)-ethoxy]-ethyl ester