A protective film consisting of an ultrathin, closely packed, and highly water-repellent two-dimensional polymer monolayer was prepared by modification of an 11-mercapto-1-undecanol (MUO) self-assembled monolayer adsorbed on copper with 1,2-bis(trichlorosilyl)ethane (BTCSE) and subsequently with octadecyltrichlorosilane (C18TCS). In order to provide regularly arranged C-18 alkyl chains and two-dimensional cross-linkages within the layer, the BTCSE-modified MUO monolayer was treated with C18TCS at low concentrations for many hours. The protective efficiency of the MUO monolayer modified with BTCSE and with 5x10(-4) M C18TCS for 24 h for copper corrosion in an aerated 0.5 M Na2SO4 was markedly high, 98.9%. The doubly modified layer was significantly water-repellent and highly protective against atmospheric corrosion of copper.