Infiltration of 53% dense preforms of 23 mu m size SiC particulate by Al-0 to 14 wt % Mg alloy melts has been carried out at different applied pressures at 750 degrees C, together with microstructural characterization of the resulting composites. The threshold pressure P* for infiltration decreased with increasing Mg content in the melt at twice the rate by which its surface tension decreased, the residual effect being attributable to the expected effect of Mg on the contact angle between Al and SiC. Periodic bands of entrapped porosity at lower Mg contents and infiltration pressures just above P* are thought to have arisen from periodic arrest of the infiltration front pending ventilation of the gas accumulated at the front. The formation of monolithic Mg2Si and areas of lamellar alpha Al-Mg2Si eutectic observed at higher Mg contents is associated with the accumulation of 5 wt % Si in the melt as a result of reaction between the melt and SiC.