We present the changes upon isochronous annealing in the room temperature photoluminescence (PL) spectra of nearly stoichiometric silicon nitride. Samples are prepared by the 50kHz PECVD of a N2/SiH4/Helium gas mixture at 350°C. In the as-deposited films the hydrogen content was around 11% for a refractive index of 1.98. For a photoexcitation at 351 nm, the polarized PL spectrum of the as-deposited film is characterized by a main broad emission band in the visible region with a maximum at 2.55eV. A second narrow peak (FWHM = 55meV), at 3.02eV is observed. We study the evolution of the PL spectra with different isochronous anneals from 350°C to 1000°C. Even though the general shape of the PL spectra does not change, we observe an increase of quantum efficiency with maximum value upon 750°C annealing. Beyond this temperature the low energy part of the PL spectrum shows a slight increase. These variations are compared to those of the infrared absorption peaks measured on the same samples. Both the temperature dependence of the intensity of a well defined absorption peak in the Si-H stretching mode region and that of the PL features can be explained assuming that solid state chemical reactions involving hydrogen and Si-Si bonds occur in the bulk of the alloy.