Mechanism of open‐circuit voltage enhancement in Au thin oxide‐n‐GaAs (1¯1¯1¯) solar cells prepared by room‐temperature wet oxidation has been studied. In contrast to earlier published results on (100) GaAs, barrier heights for metal‐insulator‐semiconductor and metal‐semiconductor cells are found to be the same, indicating absence of charge in the oxide or significant modification of surface states. It is shown that the increase in open‐circuit voltage is mainly caused by suppression of the majority‐carrier current due to tunneling through the oxide.