: Reconstruction of the Cu(001) surface due to oxygen gas impingement on a clean copper surface was directly observed by in situ UHV-TEM. Strain contrast between differently oriented surface reconstruction domains, assumed to be radical2 x 2 radical2 R45, were clearly visible by this method. The reconstruction precedes the nucleation of Cu2O islands. When Cu2O islands were partially reduced and then re-oxidized, a dwell time before formation of Cu2O was noted, demonstrating that a reconstructed Cu-O surface monolayer is necessary before oxide formation.