Abstract The aim of this work is to investigate the effect of lowering the phosphate content, from 27 to 20 mol%, in calcium-phosphate invert glass/glass-ceramic containing magnesia and niobia (MgO-CaO-P 2 O 5 -Nb 2 O 5 ) on the structure and dissolution properties. According to 31 P solid-state nuclear magnetic resonance (NMR) and Raman spectroscopies, glass containing 20 mol% of P 2 O 5 was exclusively composed of orthophosphate ( Q P 0 ), whereas orthophosphate and pyrophosphate ( Q P 1 ) coexisted in 27 mol% P 2 O 5 glass. Tetrahedral niobate was detected by Raman spectroscopy with 20 mol% phosphate, suggesting that niobate acts as a network former and therefore cross-links orthophosphate groups together (P-O-Nb bonds). The equivalent glass-ceramic consisted of crystalline phases ( β -Ca 3 (PO 4 ) 2 , Mg 3 (PO 4 ) 2 and Mg 3 Ca 3 (PO 4 ) 4 ), which did not contain niobate, and a residual glassy phase. The glass with 20 mol% P 2 O 5 showed a lower chemical durability than the glass with 27 mol% P 2 O 5 , which was likely due to the formation of P-O-Mg bonds, which favor hydrolysis. The glass-ceramic chemical durability was improved after crystallization, probably because of an increase in the niobate concentration of the residual glassy phase.