Abstract: Six differently 4-substituted 2, 2, 6, 6-tetramethylpiperidyl-N-oxides were synthesized and their ESR hfs constants aN determined in n-hexane and glycol respectively. It is shown that an approximate linear correlation between α_N and Pauling's electronegativity x of the 4-substituent is present for both solvents, but the slopes of the α_N-x straight lines are very different. Based on the EHMO calculations it is proposed that the effect of 4-substituent on aN may be caused by a kind of field effect which affects the spin density (i. e., the electron distribution in the frontier orbital) and the "out of plane angle" (i. e., the hybridization of the N atom). The solvent effect is explained in terms of the EHMO calculations, too.