The chemistry of potassium–sodium ion exchange on hydrated molybdenum bronzes (ion-exchange isotherms and structural changes during the ion exchange,etc.) was investigated. Rather complex ion-exchange behaviour, including hysteresis phenomena, was observed. The behaviour was interpreted based on a discussion of structural changes of the materials and of the solvation and coordination behaviour of the investigated ions. A variety of techniques characterized the properties of the intercalated Na and K ions or their intercalation compounds.