Compounds of nominal composition Na1-xKxV3O8 have been obtained by reacting aqueous solutions containing NaOH and KOH with V2O5. For x less than or equal to 0.4, single phases can be obtained, whereas at higher x values the bulky KC ion causes a strain which cannot be tolerated by the pristine unit cell. As a consequence, K1+rV3O8 is also formed which is not isostructural with Na1+xV3O8 and whose electrochemical performance is poorer. A compound of composition Na0.7K0.3V3O8 was characterised in detail. It can intercalate up to similar to 4 Li+ per formula unit and Li+ diffuses very fast in its enlarged layered structure. As this material is not sensitive to deep Li + intercalation, it can be cycled at high rates and depths of discharge with good capacity retention.