The nucleation and crystallographic orientation of both the magnesium matrix and the eutectic {Mg(ZnCu)(2)} at the beta-SiC particle surface were investigated. The eutectic nucleates at the particle surfaces with an identical crystallographic orientation and covers the particle surfaces in ZC63 and ZC71 magnesium matrix composites, but no distinct crystallographic orientation of the magnesium matrix/SiC particle interface has been resolved. At the interface no extensive chemical reaction takes place and no reaction product forms as a layer at the particle surfaces, but in some cases, during composite processing Mg2Si particles, and during heat treatment Mg2Si precipitates, nucleated at the particles and in the matrix close to the interface. In addition, other phases having the composition of CuFeCrSi and CuMoMgSi, the formation of which was not known in ZC63, and manganese- and iron-rich phases in ZC71 alloy, were also detected at the particle surfaces. There is some indication that the coarse and brittle manganese- and iron-containing particles may be involved in the fracture, but no direct effect of the other phases on tensile properties and/or fracture toughness was observed. Nucleation of the eutectic resulting in a good interfacial bonding, however, increases both the Young’s modulus and the ductility of the composites, modifying the brittle nature of the matrix/particle interface. As well as beta-SiC, alpha-SiC particles are intended for use in the fabrication of magnesium matrix composites. Therefore, the possibility for similar nucleation and orientation relationships between the eutectic and alpha-SiC particles has been examined.