In order to improve the formability of magnesium alloys at low temperatures, a thorough understanding of the twinning behaviour is required. Cylindrical samples were compressed uniaxially from room temperature to 250oC, with the compression axis parallel to the transverse direction (TD) of a hot-rolled Mg-3Al-1Zn sheet. Twinning was investigated by use of orientation maps constructed from electron backscattered diffraction (EBSD) data. Tensile twinning was observed at all temperatures. In some grains two twin variants were observed. For each twin the specific twinning variant among the 6 possible twinning systems was identified by consideration of the twinning rotation. The twinning was then investigated by comparing the specific twin variant with the calculated Schmid factors for all 6 possible twin variants. In cases of grains that are completely reoriented by two twin variants it is possible to back-calculate the initial grain orientation by considering all possible twin rotations for both of the twin variants. It was found that the operative twin variants where those with the highest Schmid factors.