Abstract <p>Interaction of β-sialons Si₅AlON₇ and Si₄Al₂O₂N₆ with a boron oxide was studied. Boron oxide was introduced in the form of boric acid, the acid content was set up at 0.5 and 5.0 wt %. Samples were sintered at 1650°C in the stream of N₂ for 2 h. It was found that all the samples showed higher densities against those for samples sintered with no aids, however, the densities were still lower as compared to theoretical levels. In most cases disproportionation of the basic phase into phases enriched with aluminum (Si₃Al₆O₁₂N₂) and silicon (Si₂N₂O) occurred. Moreover, when Si₄Al₂O₂N₆ was sintered with an addition of 0.5 wt % of boric acid, a noticeable amount of SiO₂ was also formed. Sintering of Si₅AlON₇ with an addition of boric acid did not lead to sufficient changes in microhardness and bending strength. In contrast, sintering of Si₄Al₂O₂N₆ sintered with an addition of 0.5 wt % of boric acid resulted in an increase in bending strength by ~14%, however, an addition of 5.0 wt % of boric acid caused a decrease in hardness by ~9% and a drop in bending strength by ~36%.</p>