CH4 and C2H2 molecules (and their interconversion) in hydrocarbon/rare gas/H-2 gas mixtures in a microwave reactor used for plasma enhanced diamond chemical vapor deposition (CVD) have been investigated by line-of-sight infrared absorption spectroscopy in the wavenumber range of 1276.5-1273.1 cm(-1) using a quantum cascade laser spectrometer. Parameters explored include process conditions [pressure, input power, source hydrocarbon, rare gas (Ar or Ne), input gas mixing ratio], height (z) above the substrate, and time (t) after addition of hydrocarbon to a pre-existing Ar/H-2 plasma. The line integrated absorptions so obtained have been converted to species number densities by reference to the companion two-dimensional (r,z) modeling of the CVD reactor described in Mankelevich [J. Appl. Phys. 104, 113304 (2008)]. The gas temperature distribution within the reactor ensures that the measured absorptions are dominated by CH4 and C2H2 molecules in the cool periphery of the reactor. Nonetheless, the measurements prove to be of enormous value in testing, tensioning, and confirming the model predictions. Under standard process conditions, the study confirms that all hydrocarbon source gases investigated (methane, acetylene, ethane, propyne, propane, and butane) are converted into a mixture dominated by CH4 and C2H2. The interconversion between these two species is highly dependent on the local gas temperature and the H atom number density, and thus on position within the reactor. CH4 -> C2H2 conversion occurs most efficiently in an annular shell around the central plasma (characterized by 1400 < T-gas < 2200 K), while the reverse transformation C2H2 -> CH4 is favored in the more distant regions where T-gas < 1400 K. Analysis of the multistep interconversion mechanism reveals substantial net consumption of H atoms accompanying the CH4 -> C2H2 conversion, whereas the reverse C2H2 -> CH4 process only requires H atoms to drive the reactions; H atoms are not consumed by the overall conversion.