Inviscid numerical simulations have been used to identify appropriate experimental test conditions to study thermochemical nonequilibrium expansions using nozzle, unsteady, and Prandtl–Meyer expansions, and using NO TDLAS for diagnostics. The nozzle expansion is the only experimental method that allows NO TDLAS diagnostics under high-enthalpy conditions where significant N2 dissociation is present. For the test condition of the unsteady and Prandtl–Meyer expansions, significant dissociation of only O2 is present. Two absorption lines, centered at around 1974.03 cm−1 and 1883.59 cm−1, have been selected for the NO TDLAS in the current work using the two-line method to determine the rotational and vibrational temperatures. The selection is made based on the peak fractional absorption being at least 0.01 in essentially all the experiments and the ratio of the integrated absorbances having reasonable sensitivity to changes in the rotational and vibrational temperatures for the range of conditions concerned in the current work.