The v(1) fundamental vibrational mode of the C5S carbon-sulfur chain has been observed at high-spectral resolution in the gas phase for the first time. C5S was produced using laser ablation of carbon-sulfur targets and observed in a free-jet expansion using quantum cascade laser spectroscopy. The v(1) mode is found centered at 2142.3072(1) cm(-1), blueshifted by some 4 cm(-1) from the v(4) vibrational fundamental of the C-7 pure carbon cluster which is also observed under the same experimental conditions. The present study is complemented by high-level CCSD(T) calculations revealing very good agreement between calculated and experimental molecular parameters. (C) 2017 Elsevier B.V. All rights reserved.