A combination of millimeter-wave and high-resolution infrared data is used to analyze the eight lowest-energy vibrational states of benzonitrile (C6H5CN, C2v, μa = 4.5 D), a benzene derivative recently detected in the interstellar medium. The overtone states v22 = 2 and v33 = 2, combination state v22 = 1, v33 = 1, and fundamental states v21 = 1 and v15 = 1 are studied for the first time by rotationally resolved spectroscopy. The three former states form a Coriolis- and Darling-Dennison-coupled triad of interacting states for which the coupling terms and highly precise, deperturbed energy separations have been measured. The use of sub-millimeter and far-infrared data together enabled the determination of the purely rotational and coupling parameters for the six lowest-energy vibrationally excited states of benzonitrile, along with their highly precise energies (E22 = 141.4810252 (57) cm-1, E33 = 160.5891953 (47) cm-1, E2×22 = 282.6295417 (83) cm-1, E22+33 = 302.5795909 (87) cm-1, E2×33 = 321.4923856 (77) cm-1, E21 = 372.257993 (10) cm-1). These energies, the resultant experimental anharmonicity constants (x22,22 = − 0.1663 cm-1, x33,33 = 0.1570 cm-1, and x22,33 = 0.4909 cm-1), and semi-experimental harmonic frequencies (ω22 = 142.9 cm-1 and ω33 = 161.0 cm-1) for the ν22 and ν33 states are compared to CCSD(T)/ANO1 predicted values. The spectroscopic and coupling constants determined in this work for the vibrational ground state, the two lowest-energy fundamental states, and the corresponding first overtone and combination states successfully predict experimental frequencies down to 8 GHz. Particularly for the vibrationally excited states, the ability to predict transition frequencies so far outside the frequency region in which the constants were determined confirms that the rotational and distortion constants, as well as the coupling terms, are determined reasonably close to their true values. The ability to accurately extrapolate also demonstrates the suitability of the determined constants as the basis for extraterrestrial identification and examination of these vibrational states of benzonitrile.