Dissociative electron capture mass spectra in the range 0-10 eV were registered for the antiviral drugs acridanone and acridanoneacetic acid and its n-hexyl and benzyl esters. The appearance energies (AEs) of fragment negative ions from these and some model compounds (carbazole, acetic acid) were measured. The following thermochemical data were deduced from experiment: electron affinities (EAs) 2.44, 3.08 and 4.12 eV for carbazolyd, 8-oxyacridane and acridanone-N-methyleneacetoxy free radicals, respectively, and DH acid values 324.5 and 320.7 kcalÁmol À1 for acridanoneacetic and 8-hydroxyacridane, respectively. The data for CH 3 COOH were: DH acid (CH 3 COOH) = 343 AE 1 kcalÁmol À1 and EA(CH 3 COO Á ) = 3.31 AE 0.05 eV. Intense formation of RCOO À carboxylate ion from acetic acid, acridanoneacetic acid and its benzyl ester contrasted with the as yet unexplainable absence of this ion in the mass spectra of CH 3 COOEt and of the nhexyl ester of acridanoneacetic acid. It was demonstrated that the [MÀH] À1 ion from acridanone possesses the structure of acridane-8-oxy anion rather than N-centred acridanonide anion. The appearance of some fragment ions at unexpectedly low AEs ( 0) was explained by their formation from free radicals produced thermocatalytically in the instrument. The estimation schemes for the enthalpies of formation of more than 20 free radicals and anions, based on the earlier established regularities, 1,2 are described in detail. Some of the data for known species were revised: DH 0 f (HCCO Á ) and DH acid (HCCOH) 48.5 and 350 kcalÁmol À1 (compare with 42.4 and 365 in Ref. 3) or EA C 4 H 4 N Á (pyrrolide) 1.53 eV (2.4 eV, Ref. 3) Copyright # 1999