[en] This paper reports the microstructures of simulated tailored ceramic forms for the Idaho Chemical Processing Plant (ICPP) high-level waste characterized by TEM. The ceramic forms were consolidated from simulated ICPP waste calcines with either silica-yttria-based or silica-lithia-based additives by hot isostatic pressing. The hot isostatically pressed ceramic waste forms are composed of cubic CaF₂, monoclinic ZrO₂, stabilized cubic ZrO₂, tetragonal ZrSiO₄, and amorphous silicate phases which can be phase separated. The phase-separated glass is a result of phase immiscibility in the soda aluminosilicate glass to the compositions of albite and mullite