IR-vis sum-frequency generation spectroscopy is used to study the stretching vibration of hydrogen chemically bound to a monolayer graphene sheet prepared on an Ir (111) substrate. A line characteristic for sp(3) bound hydrogen is observed when a mixture of H and D is dosed. The intensity per surface H oscillator is largest when H is strongly diluted in D. This is interpreted to indicate that graphane is formed by concurrent attachment of hydrogen to graphene from the gas phase and from intercalated hydrogen. In this case isotope mixing is a prerequisite for SFG activity. (C) 2012 Elsevier B.V. All rights reserved.