Abstract. The XI2 solar flare that occurred on 1991 June 11 was well observed by the instruments on CGRO. Here we present observations of this flare obtained with OSSE. OSSE detected nuclear y-ray lines, 2.223 MeV neutron-capture radiation, 0.511 MeV positron-annihilation radiation, >16 MeV y rays and neutrons. We apply three techniques of measuring the accelerated-particle energy spectrum. Since each technique is sensitive to a different range of particle energies, comparison of the derived spectral indexes provides information about the shape of the particle spectrum over a broad energy range. We show that as the flare progressed, the particle spectrum clearly evolved. There were intervals when the particle spectrum was consistent with an unbroken power law from 1 to 100 MeV. But there were also intervals when the spectrum deviated significantly from a simple, unbroken power law through this energy range
