This work investigates the structure of phosphosilicate glasses from the SiO2-MgO-K2O-3CaO-P2O5 system, as determined by Si-29 and P-31 MAS-NMR. According to the Si-29 NMR, the actual network connectivity of the glasses generally exhibits a tendency for decreasing with increasing the alkali additive content. P-31 NMR shows that the glass structure includes orthophosphate groups (PO43-) closer to Ca2+ ions than to Mg2+ ions, which are probably deeply inserted within the silicate network. Structural features were related with acellular bioactivity performance, in terms of in vitro capability of glass surface for inducing apatite mineralization in simulated body fluid. Needle-like apatite precipitates at the surface of the investigated bulk glass samples. A relatively flat and poorly crystallised hydroxyapatite (HAp) layer forms on the K-enriched glasses. For glasses without or with smaller K content, nearly amorphous octacalciumphosphate (OCP) seems to predominate over HAp. In this case apatite balls, built up from thin needles, are spread over the glass surface. Both the crystallinity degree and the thickness of the apatite precipitates, as observed by SEM, seem to evolve with increasing K content, consistently with the decrease in the average network connectivity detected by NMR.