Summary: Unit cell dimensions and refractive indices have been determined for synthetic pyroxenes in the system CaMgSi₂O₆-CaFeSi₂O₆-NaFeSi₂O₆, which have been crystallized and annealed on the nickel-bunsenite (Ni-NiO) oxygen buffer curve at 700 °C and a total pressure of 2 kb. The data presented confirms the many postulates that a complete solid solution series does exist in the subsolidus region of the ‘ternary’ system. The unit cell dimensions show an almost linear change with composition between the end members. The determined cell parameters for a series of analysed natural alkali pyroxenes have been compared with the data for the synthetic pyroxenes and close agreement was found between both sets of data. The variation of the unit cell dimensions and refractive indices has been examined in relation to the possible ionic substitutions taking place in the pyroxenes.