We present a time dependent analysis of the &ggr;‐ray line spectra recorded by the PHEBUS instrument on board GRANAT during the 1991 June 1 flare. For each studied spectrum we determine the electron bremsstrahlung continuum and the &ggr;‐ray line fluences &Fgr;1.1–1.8and &Fgr;4.1–7.6, in the 1.1–1.8 MeV and 4.1–7.6 MeV bands respectively, as well as their ratioR=&Fgr;1.1–1.8/&Fgr;4.1–7.6. We find that the power law index of the bremsstrahlung continuum in the 0.6–10 MeV band is 2.6±0.1 and that it remains constant with time. The characteristic value ofR, estimated for the spectrum accumulated over the whole event duration is about 4 but, surprisingly,Rincreases from a value of ∼3 to a value of ∼7 during the decay phase of the event. In order to interpret these findings we have compared the measured values ofRwith those obtained by using calculations of deexcitation &ggr;‐ray line spectra performed for different interacting particle spectra and compositions. We show that the high value ofRand its large variation with time are inconsistent with a thick target &ggr;‐ray line source and that a thin target model is the appropriate one. This is consistent with an earlier study of the 1991 June 1 event which indicates that the observed nuclear line emission is produced in a coronal source. We finally argue that the increase ofRwith time strongly suggests an impulsive flare composition for the accelerated particles and the further enrichment of the heavy nuclei abundances as the flare progresses. For thin target interactions such an enrichment is most probably due to the acceleration process itself. ©1996 American Institute of Physics.