In this thesis work, the results of microwave investigation and structural determination for benzonitrile and some of its fluorinated derivatives are presented. The pure rotational spectra of the studied compounds including benzonitrile, 2-fluorobenzonitrile, 3-fluorobenzonitrile, 2,3-difluorobenzonitrile, 2,4-difluorobenzonitrile and pentafluorobenzonitrile were investigated. Measuring the rotational spectra of the parent molecules and the minor 13C and 15N isotopic species allowed the derivation of the substitution and effective structures for these molecules. Using the effective and the calculated ab initio structures, the geometries of the fluorinated derivatives were compared to that of the reference compound (benzonitrile) and the effect of single, double and full fluorination on the geometry of benzonitrile was examined. The observed distortions in the BN geometry caused by single, double and full fluorination were interpreted by hybridization theory and intramolecular non-bonded interactions. / February 2015