In the current study, crystallization and phase separation of a borosillicate glass composition in the system Na2O–CaO–Al2O3–B2O3–SiO2 used for nuclear waste immobilization have been investigated. At first, base glass composition was prepared by mixing and homogenizing of pure raw materials, melting in an alumina crucible and then casting in a stainless steel mold. Mo2O3 was added in constant values of 0.5, 1.5, 2.5 and 3.5 %mole. XRD analysis was carried out to ensure no crystalline phases were formed. Heat treatment was performed at temperature intervals of 620, 650, 680 and 710°C. To study phase separation creation, SEM analysis was carried out. In the next step, acid leaching of phase separated glasses was performed. Porosity analysis and measurement of specific surface area by BET were performed on acid-leached samples. The results show that all samples show different amounts of phase separation. Also, the highest specific surface area was related to the sample containing 1.5 %mole Mo2O3 with two acid washing steps.