Quantum chemical calculations of 2-, 3-, and 4-phenyl substituted pyridine-N-oxide derivatives revealed that 2- and 3-phenyl derivatives nitrate via free base mechanism. For 4-phenylpyridine-N-oxide, however, calculations imply conjugated acid mechanism for nitration. For 2- and 4-benzylpyridine-N-oxide molecules a conjugated acid mechanism were suggested in nitration process. Attempts to search a correlation between the experimental acidity constants, pK(a), and computed acidity constants were successful. (C) 2003 Elsevier B.V. All rights reserved.