This paper discusses the joining of AZ91 magnesium alloy with AlSi17 aluminium alloy by compound casting. Molten AZ91 was cast at 650°C onto a solid AlSi17 insert placed in a steel mould under normal atmospheric conditions. Before casting, the mould with the insert inside was heated up to about 370°C. The bonding zone forming between the two alloys because of diffusion had a multiphase structure and a thickness of about 200 μm. The microstructure and composition of the bonding zone were analysed using optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. The results indicate that the bonding zone adjacent to the AlSi17 alloy was composed of an Al3Mg2 intermetallic phase with not fully consumed primary Si particles, surrounded by a rim of an Mg2Si intermetallic phase and fine Mg2Si particles. The bonding zone near the AZ91 alloy was composed of a eutectic (an Mg17Al12 intermetallic phase and a solid solution of Al and Si in Mg). It was also found that the compound casting process slightly affected the AZ91 alloy microstructure; a thin layer adjacent to the bonding zone of the alloy was enriched with aluminium.