Photoinduced reactions of trans-HC=CH- in a glassy 2-MTHF matrix were studied by ESR and electron absorption (EA) spectroscopy. ESR and EA spectra were interpreted with the aid of the results of molecular orbital (MO) calculations. The trans-HC=CH- was generated and stabilized in the 2-MTHF matrix at 77 K by ionizing irradiation. The ESR parameters observed for the radical anion are A(1) = 5.3, A(2) = 4.6, A(3) = 4.5 mT for two equivalent H-1 atoms, A(perpendicular to) = 1.5, A(parallel to) = 4.2 mT for two equivalent C-13 atoms, and g(1) = 2.0009, g(2) = 2.0024, g(3) = 2.0029 for the g tensor. By illuminating with light of lambda greater than or equal to 430 nm, the trans-HC=CH- was found to be irreversibly isomerized into the vinylidene radical anion, H2C=C-, in the matrix. The experimental ESR parameters of H2C=C- [A(1) = 5.4, A(2) = 6.0, A(3) = 5.7 mT for two equivalent protons, g(1) = 2.0026, g(2) = 2.0008, g(3) = 2.0026] were fully consistent with the results obtained by semiempirical MO methods, (1) INDO and Analytical Dipolar Calculation method (ANADIP) for isotropic and anisotropic H-1 hyperfine (hf) couplings, respectively, and (2) AM1 for the g tensor, for a geometrical structure optimized by ab initio method (Gaussian90/uhf/6-31++G**). The structure of the anions was in agreement with the results obtained from the EA spectroscopic study. The photoinduced isomerization was initiated by the light absorption at lambda greater than or equal to 430 nm of trans-HC=CH-. The EA band at lambda(max) = 374 nm observed for H2C=C- was attributed to the electron transition from the B-2(2) ground state and to the (2)A(1) excited state.