The precipitation behavior of Mg-Al alloys during isothermal ageing has been investigated by dilatometry. Only continuous precipitation was observed in Mg-5.5Al during ageing at 175°C, 200°C, or 225°C, while both discontinuous and continuous precipitation occurred in Mg-8.9Al at the same ageing temperatures. Dilatometry curves showed that Mg-8.9Al expanded when aged at 225°C or 250°C. These results demonstrate that high content of Al is beneficial for discontinuous precipitation in binary Mg-Al alloys. Additional analyses indicated that the expansion of Mg-8.9Al during isothermal ageing can be attributed to the increment of the Mg cell lattice parameters. Moreover, continuous precipitation could suppress the discontinuous precipitation by reducing the driving force for growth of discontinuous precipitates.