A novel method is exploited in this report to directly determine the relative detection sensitivity of the (2+1) resonance-enhanced multiphoton ionization (REMPI) bands of CH3 and CHD2 radicals. The basic idea is based on the simple fact that in an infrared (IR) absorption process the number of molecules being pumped from the lower state must be the same as the number of molecules in the excited upper state. Hence, the measured intensities of the respective REMPI bands should directly reflect their relative detection sensitivities. In order to ensure the processes involved and better quantify the measurements, extensive IR-UV double resonance spectroscopy was also performed. Using the REMPI-IR scheme, the IR spectrum of the vi fundamental (CH stretch) of CHD2 was obtained and assigned for the first time. Using the IR-REMPI approach, high-resolution (2+1) REMPI spectra via the Rydberg 3p states of both radicals were demonstrated in a rotationally specific manner for both the origin and vibronic-excited bands, from which the predissociation rates of the Rydberg 3p states were deduced.