An experimental study of a long-lived unstable electron belt created on March 24, 1991 is presented. The new belt occupied the slot region between the inner and outer radiation belts. Electrons, positrons, and protons with 20--200 MeV/c momenta were measured with the Mariya-2 magnetic time-of-flight spectrometer-telescope on board the orbital station Mir. The unstable belt turned out to consist solely of electrons (the positron fraction is less than 1%). The electron energy spectrum of this unstable belt has been analyzed throughout its 2-year lifetime. Within the energy range 10--50 MeV, the spectrum agreed well with a power law E-&agr; with &agr;=13¿5. This spectrum is very soft compared with the spectrum of the stable electron belt located in the inner zone. ¿ 1999 American Geophysical Union