Magnesium alloy AZ91 (9 % Al, 1 % Zn, 0.2 % Mn in wt.%) with different reinforcements has been used to study fracture mechanisms and crack development. SiC particles and/or Saffil fibres were used as the reinforcement. Fracture surfaces of specimens prepared by impact tests in the temperature range from room temperature to 300 degrees C were investigated by scanning electron microscope (SEM). Possible mechanisms of fracture are discussed in the relation to the test temperature.