The first optical observation of a mixed germanium-silicon-carbon (GelSimCn) cluster has been made by trapping the products of the laser ablation of carbon/silicon/germanium rods in Ar at 10 K. The nu(1)(sigma) C-C stretching mode of linear GeC3Si has been identified at 1939.0 cm(-1). The assignment is confirmed by the very good agreement between Fourier transform infrared (FTIR) measurements of frequencies and isotopic shifts and predictions of our quantum theoretical calculations.