High-resolution infrared absorption spectroscopy of acetylene gas is demonstrated in dispersion-engineered photonic crystal waveguides under slow light propagation. Individual absorption profiles are obtained for both TE and TM polarizations for group indices ranging from 1.5 to 6.7. Experimental enhancement factors of 0.31 and 1.00 are obtained for TE and TM polarization, respectively, and are confirmed by time-domain simulations. We experimentally demonstrate that molecular absorption is a function of the electric field distribution outside the photonic crystal slab and the group index under structural slow-light illumination.