A new voltage step at 4.8 V can be activated in manganese based cobalt free phospho olivines by activating the utilisation of the Mn3+/4+ redox pair applying Mg2+ as a Li+-storage agent. CCCV cycling reveals a significant excess in discharge capacity of Mn–Mg co-containing samples related to the theoretical single electron step of Mn2+/3+ and the occurrence of a new potential step at 4.8 V. These phenomena are not observed in Mg- or Mn-free samples even after charging them up to 5.3 V vs. Li/Li+. Similar observations are made for Ni–Mn containing samples. Mg–Mn containing cathode materials can be a first step towards cobalt free high voltage phosphates for Lithium Ion batteries.