In this work results are presented of various experiments involving the amorphous form of massive selenium, and a structural study performed by X-ray diffraction in the wave vector region 0<K<17.5 A ̊ and over a large range of temperature (78 to 683 K). At first, to suppress doubts which subsist in spite of several published reports concerning amorphous material, we examine in detail the dependences of the physical properties and the structural variations on applying a uniaxial compression at 293 K in the metastable form. Comparisons between deformed or non-deformed materials (D a-Se or ND a-Se) are made using measurements of density, longitudinal sound velocity, differential thermal analysis, small angle scattering and careful analysis of the structure factor J( K). The information gained on the structural evolution with temperature is discussed. The variation of Debye temperature, determined from measures of sound velocity between 78 K and 300 K, permits us to calculate the thermal attenuation. In amorphous selenium, the Debye hypotheses are not valid. A temperature variation has no influence on the structure of the amorphous phase. In the viscous phase a slow evolution is observed but only in the liquid phase does a real change of J( K) appear. A disordered chain model is retained to describe the structure of the molecule in amorphous, viscous and liquid selenium. The elementary triangle of the selenium lattice remains constant. The intramolecular deformation is due to a limited variation of the dihedral angle which is random in sign. The limits of variation are reduced weakly by a uniaxial compression and also by an increase of temperature but only for T>T g; the deformed chains approximate to the hexagonal arrangement. The presence of prolate ellipsoidal voids included between the chains explains the difference in density between the crystallized hexagonal Se and the amorphous material. Finally several simulated models are compared with the experimental J( K) measured at different temperatures. The same uncertainty always remains in the description of the models and the explanation of experiments as a result of the impossibility of separating the two contributive intra and intermolecular disorders.