<p>Borosilicate glasses used in nuclear waste immobilization suffer from low solubility of Mo, S, and some rare-earth elements, thereby limiting overall waste-loading capacity. A simulated nuclear waste stream was loaded into an aluminoborosilicate glass with the addition of low levels of phosphorus. The morphology, elemental distribution, and chemical durability of the vitrified wasteforms were characterized as a function of waste loading. The presence of phosphorus induces glass-in-glass phase separation while improving the overall incorporation and retention of high field-strength cations in the composite wasteform. Waste loading of 15 wt% resulted in the greatest chemical durability in aluminoborosilicate glasses doped with 5&#xa0;mol% P₂O₅. Increasing phosphate beyond this level significantly decreased the durability of the wasteform. This work demonstrates the potential value of phosphorus for improving the vitrification and retention of high field-strength cations in nuclear waste immobilization, without impeding the formation of a silicate-rich alteration layer.</p> Graphical abstract <p></p>