This work reports the reliable experimental observations of the simplest negative triatomic ions, H-3(-) and D-3(-) anions, the stability of which has been debated for several decades, from dielectric barrier discharge hydrogen and deuterium plasmas. The observed H-3(-) and D-3(-) mass signals with widely distributed kinetic energies, from a few eV to similar to 100 eV, are contributed from their ground states with lifetimes greater than 8-35 mus. The agreement between theoretical and experimental results for H-3(-) has been reached. The three-body collision process is proposed to be the dominant formation mechanism for the H-3(-) and D-3(-) anions. (C) 2003 Elsevier B.V. All rights reserved.