Abstract The long range potential fluctuations (LRPFs) arising from the defects and heterogeneities in disordered semiconductors are important for understanding their atomic and electronic properties. Here, they are measured in GeXSbYTe1−X−Y (GST) chalcogenide glasses used in rewritable phase change memory (PCM) devices. It is found that the most commonly used composition Ge2Sb2Te5 has the smallest LRPF amongst its nearby compositions. This finding may be useful in the search for better PCM materials.