A system to measure time-resolved Fourier-transform infrared absorption spectra of gaseous samples using a commercial step-scan spectrometer is described. To increase the signal intensity, the incident infrared light is multipassed within a White cell. Light from a photolysis laser passes through the reaction cell to initiate the reaction in the flowing gaseous sample. The variation of absorbance is obtained from the ac-coupled signal whereas phase information and a reference spectrum are from the de-coupled signal. The system is tested by probing the temporal evolution of HCl(v) in the chain reaction of H-2 and Cl-2 initiated by photolysis at 355 nm. Time-resolved absorption spectra of HCl(v = 0-2) were obtained with spectral resolution 0.75 cm(-1) and intervals down to 5 mu S. Kinetic modelling of deduced temporal profiles of HCl(v=0-2) yields rate coefficients of (1.38+/-0.04)X10(-14) and (5.8+/-0.4)X10-(15) cm(3) molecule(-1) s(-1) (in which error limits represent only the uncertainty of the fit) for reactions Cl+H-2 --> HCl(v=0)+H and Cl+H-2 --> HCl(v=1)+H, respectively; the total rate coefficient is in agreement with previous kinetic measurements. (C) 1997 American Institute of Physics.