Platinum-rich aluminium/platinum alloys have been synthesised via a coupled reaction between aluminium oxide and platinum in a stream of hydrogen at temperatures in the region of 1200 degrees C. In accordance with the composition, the platinum sites are statistically occupied to a slight extent by aluminium atoms. Upon heating the alloys containing a slight tetragonal distortion, a second order phase transition into a cubic phase occurs whilst the alloys with a stronger tetragonal distortion undergo a corresponding phase transition which is first order. This result conforms with the idea that, in the former case, the distortion of the platinum octahedra is progressively relieved with increasing temperature while in the second case this event results discontinuously.