1. The mass spectra of nitroxide biradicals with a system of σ- and σ -π-bonds were studied. 2. For three 2,2,6,6-tetramethylpiperidinyl-1-oxyls, connected by a system of σ-bonds, in electron impact no interaction of the radical centers is observed, while in the case of Δ3-unsaturated 2,2,6,6-tetramethylpiperidinyl-1-oxyls, bonded through two acetylene groups, an interaction is observed between the two radical centers, which leads to the appearance of peaks of the ions [M-NO-CH2O]+ and [M-2CH3O]+ in the mass spectrum of this compound. © 1975 Plenum Publishing Corporation.