Decay processes of H-2(-) anions in gamma-ray-irradiated solid parahydrogen were studied by using an ESR spectrometer. The following interesting results were obtained. First, the initial yields of ortho-H-2(-) anions in the gamma-irradiated solid parahydrogen were 3 times as large as those of para-H-2(-) anions. Second, the yields of para-H-2(-) anions decrease faster than those of ortho-H-2(-) anions upon storage of the irradiated samples at 4.2 K. Third, the decay rate of H-2(-) anions is accelerated by the addition of HD and D-2 molecules. Fourth, H-2(-) anions at 2.2 K decay faster than at 4.2 K. According to the parity conservation rule in a homonuclear diatomic molecule, the energy of ortho-H-2(-) anions at the ground state is lower than that of para-H-2(-) anions, whereas that of ortho-H-2 molecules is higher than that of para-H-2 molecules at low temperatures. The first and second results are ascribed to para --> ortho conversion of H-2(-) anions. The third result indicates that nearby HD and D-2 accelerate the decay of H-2(-) anion or broaden the spectra by superhyperfine interactions. The fourth result suggests a coherent tunneling mechanism of decay of H-2(-) anions in the solid parahydrogen.