Several novel benzimidazole-3-oxide-1-oxyl radicals with substituents at 5 and/or 6 position were synthesized. The ESR analysis of nitrogen hyperfine coupling constants (hfccs) revealed that substituents at 5 and 6-position affect the spin density to greater extent than substituents on the phenyl ring at 2-position. Density functional theory calculations of nitrogen hfccs were performed using several different Pople type basis sets, as well as double and triple zeta quality individual gauge for localized orbital (IGLO-II, IGLO-III) and electron paramagnetic resonance (EPR-II, EPR-II) basis sets. Experimental and theoretical hfccs are compared. Copyright (C) 2009 John Wiley & Sons, Ltd.