The spectral dynamics of a mid-infrared multimode Cr2+:ZnSe laser located in a vacuum sealed chamber containing acetylene at low pressure is analyzed by a stepping-mode high-regolution time-resolved Fourier transform interferometer. Doppler-limited absorption spectra Of C2H2 in natural isotopic abundance are recorded around 4000 cm(-1) with kilometric,absorption path lengths and sensitivities better than 3 x 10(-8) cm(-1). Two cold bands are newly identified and assigned to the v(1) + v(4)(1) and v(3) + v(5)(1) transitions of (CCH2)-C-12-C-13. The v(1) + v(5)(1) band of (C2HD)-C-12 and 14 (C2H2)-C-12 bands are observed, among which for the first time v(2) + 2v(4)(2) +v(5)(-1). (c) 2005 Elsevier B.V. All rights reserved.