The acidity-basicity behaviours of some 2,3 and 4 methoxy, methylthio and cyano substituted pyridine derivatives were investigated using DFT method. The correlation attempt between the experimental and calculated acidity constants, pK(a) values, revealed that the DFT calculated values are much closer to the experimental values with a regression of almost unity (R-2 = 0.99). A strong geometry dependence of experimental acidity constants, pK(a), was observed. The basicity-nucleophilicity correlation attempt between experimental acidity constants, pK(a), DFT computed nucleophilicities, n, was unsuccessful. The correlation between the calculated electronic charges and experimental pK(a) values was unexpectedly high. (c) 2005 Elsevier B.V. All rights reserved.