Self-assembled monolayers (SAMs) of 11-mercapto-1-undecanol (TOH) and mixed monolayers of TOH and dodecanethiol (TCH3) were transformed to isocyanate-bearing layers by reaction of the hydroxyl groups in the monolayers and mixed layers with 1,4-phenylene diisocyanate (PDI). PDI reacts with only one of its isocyanate groups, yielding a carbamate (urethane) group; conversion of the hydroxyl groups is nearly quantitative, and the attached phenylene isocyanate moiety is oriented perpendicularly to the surface, as indicated by IR reflection measurements at grazing incidence. The free isocyanate groups readily react under mild conditions with a variety of substances, for example, water, alcohols, and amines. Hence, the conversion of surface hydroxyl groups with PDI offers a versatile and straightforward method to modify surfaces and to facilitate further reactions with additional compounds.