High-resolution infrared absorption spectroscopy of acetylene gas is demonstrated in a dispersion-engineered photonic crystal waveguide under slow-light propagation. Experimental enhancement factors of 0.31 and 1.00 are obtained for TE and TM polarization, respectively, for group indices ranging from 1.5 to 6.7. The dependence of molecular absorption on the evanescent electric-field distribution and on the group index under structural slow-light illumination is experimentally demonstrated and confirmed by time-domain simulations.