The ground state rotational spectra of a series of fluorinated benzonitriles (BN), namely: 2-fluorobenzonitrile (2FBN), 3-fluorobenzonitrile (3FBN), 2,3-difluorobenzonitrile (23DFBN), 2,4-difluorobenzonitrile (24DFBN) and pentafluorobenzonitrile (PFBN), have been investigated between 4 and 24 GHz using Fourier transform microwave (FTMW) spectroscopy. The assigned transitions include those due to the parent as well as the 13C and 15N singly-substituted isotopologues which were observed in natural abundance. The spectroscopic analysis allowed the derivation of substitution (rs) and effective ground state structures (r0) to investigate the effect of mono-, di- and pentafluoro substitution on the geometry of the BN backbone and are compared here with ab initio values of the equilibrium parameters (re) obtained from MP2/6-311++G(2d,2p) calculations. Analysis of the 14N hyperfine structure provides additional information about the electronic structure surrounding the nitrogen atom. The observed geometry changes relative to the reference BN compound are interpreted using natural bond orbital (NBO) analysis to describe differences in the hybridization at various sites and contributions from plausible resonance structures.