Previous reports on the solubility of magnesium in B'-sialons have been conflicting. The present work shows conclusively that metastable crystalline B"-magnesium sialons do exist and can be formed by two distinct crystallization mechanisms: (1) Low temperature annealing of quenched MgSiAlON (3M/4X) glasses containing precipitated B-Si3N4/B' sialon nuclei which must be present for epitaxial nucleation and growth to proceed. The degree of B" crystallization which can be achieved is sensitive to both composition and impurity levels and complete crystallization (>90%) can be achieved only for compositions within the 3M/4X plane using high purity starting materials. 2. Coupled growth with forsterite from MgSiAlON (3M/4X) liquids producing low magnesium content B" in a 'eutectic' microstructure. B"-magnesium sialons have a range of compositions within the 3M/4X plane of the Mg-Si-Al-0-N system, with compositions of high magnesium content tending to exist in a limited composition range indicative of a 'phenacite' type metal atom ordering between (Mg,Al) and Si, as confirmed by the existence of additional superlattice reflection in electron diffraction photographs. The metastable nature of B"-magnesium sialons means that they tend to transform to stable crystalline products at high temperatures. The range of B" composition gives a corresponding range of thermal stability, with high magnesium content B" stable only up to 1120°c, while B" crystals with lower magnesium levels are able to exist up to 1350°c. A previously unknown MgSiAlON phase, termed Q-phase, has been crystallized as a secondary phase from MgSiAlON liquids in the 3M/4X plane in association with forsterite.