We have used CW cavity-ring-down-spectroscopy around 1.5 mu m to probe a free supersonic expansion of mixed acetylene and Argon. The high resolution spectrum of the 2CH band in the (C2H2)-C-12-Ar van der Waals complex was recorded, at a rotational temperature of about 15 K. Upper state spectroscopic constants are provided from the fit of the 44 assigned vibration-rotation lines in the K-a = 0 <- 1 and 1 <- 0 sub-bands to a conventional Watsonian. (C) 2007 Elsevier B.V. All rights reserved.