The microstructure of three dilute Mg-Mn and two dilute Mg-Zr alloys which had been heat treated at different temperatures and times were studied using optical microscopy and various electron optical techniques, including FEGSEM and TEM. It was found that the Mg-Zr alloys homogenized at 500 degrees C and annealed at 350 degrees C contained a fine dispersion of Zr-containing particles at grain boundaries and within grains. In contrast, annealing Mg-Zr alloys at 300 degrees C for up to 3 h led to little modification of the as-cast structures. When the Mn content was less than 0.9 weight percent, homogenization of the Mg-Mn alloys at 550 to 600 degrees C resulted in the dissolution of small rod-like and needle-shape particles, which then grown up as fine particles when aged at 300 degrees C. Mg-0.6Mn and Mg-0.9Mn alloys annealed at 300 to 400 degrees C without solution treatment contained a large volume fraction of nano-sized precipitates.