A new experimental setup for in. situ studies of the metal/atmosphere interface has been developed based on simultaneous infrared reflection absorption spectroscopy (IRAS) and quartz crystal microbalance (QCM) measurements of a metal surface. It consists of an in situ chamber in which the metal can be exposed to a well-controlled atmosphere. Four external devices are connected to the in situ chamber; a Fourier transform infrared spectrometer with external optical compartments, a QCM sensor probe with a frequency counter, a corrosive air generator, and a corrosive air analyzing system. In order to demonstrate the capability of the IRAS/QCM setup, copper was exposed to purified air at 80% relative humidity and 25 degrees C. Under these exposure conditions, the interface between copper and air consists of cuprous oxide and water physisorbed on the oxide. The kinetics of the cuprous oxide formation could be followed in situ with both techniques. The combined IRAS/QCM results show excellent agreement with previous combined IRAS and cathodic reduction measurements and with optical calculations of the IRAS response. Under these conditions, the detection limit in terms of an equivalent Cu2O film thickness is 10 Angstrom for IRAS in situ analysis and 2 Angstrom for QCM in situ analysis, respectively.