Аннотация:Acidic properties of several compounds with aminoxyl-based spin labels were found to be abnormally strong when compared with those of diamagnetic analogues. Analysis of this phenomenon in terms of both Taft constants and Hinze-Jaffe-Huheey group electronegativities enabled us to attribute it to the strong electron-acceptor effect of the aminoxyl group that results in high electronegativity of aminoxyl-containing substituents. Quantitative estimates of this effect for two spin labels were obtained. Group electronegativity of imidazoline-based spin label is close to that of the -CF3 substituent, while the electron-acceptor effect of piperidine-based spin label is intermediate between those of -CHF2 and -CH2F groups. The strong electron-acceptor effect of aminoxyl spin labels should be taken into account when using them in biological and other studies.