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Reverse water-gas shift (RWGS) reaction over La$2$NiO$4$ catalyst was investigated during CH$4$/CO$2$ reforming by continuous wave (cw) cavity enhanced absorption spectroscopy (CEAS), a sensitive absorption technique. A cw-diode laser with wavelength in the 1.3 $\mu$m region and an optical cavity consists of a pair of high reflectivity mirrors (0.99995) were used in the experiment. Absorption lines of the $\Delta J=0$ (Q branch) rotational transitions of the 2$_0^1$3$_0^1$ vibrational transition of CH$4$ near 7510 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, and the $J=5,Ka=0,Kc=5$ to $J=6,Ka=2,Kc=4$ rotational transition of the 1$_0^1$3$_0^1$ vibrational transition of H$2$O near 7511.29 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ were monitored. The results indicated that the RWGS reaction promoted the conversion of CO$2$ and decreased the partial pressure of hydrogen. Our CEAS result confirms that the conversion of CO$2$ is always 3-5% higher than CH$4$ due to RWGS reaction. In addition, the effect of reaction temperature on RWGS was studied. The work described here was supported by a grant from the Research Grants Council of Hong Kong SAR, China (Project No. HKU7101/02P)