To obtain the subject amorphous form and crystal modification having excellent density, flowability and diffusibility, useful as a stabilizer for processing an organic polymer, etc., by melting a specific phosphite compound, rapidly cooling it into an amorphous shape, heating it to a high temperature and crystallizing it.

1,1',1''-Nitrilo{tri-2-propyl-tris-[2,2'-ethylidene-bis(4,6-di-tert- butylphenyl)] phosphite of the formula is melted, rapidly cooled into an amorphous modification having a glass transition temperature (Tg) in the range of 137-142°C (DSC) and a featureless X-ray diffraction pattern. The melt of the compound is heated to 170-200°C under 400-0.1 mm Hg reduced pressure and is crystallized in a new crystal modification. The crystal modification has 254-274°C melting point and an X-ray diffraction pattern using Cu-Kα in which the angle of diffraction (2 θ) show extremely high intensity lines at 7.9 and 15.6, high intensity lines at 7.3, 8.9, 14.7, 16.8, 18.3, 18.9, 19.8 and 20.6, medium intensity lines at 13.9, 22.6 and 24.0 and a low intensity line at 21.4.