Hydrogen atoms are detected in situ in atmospheric pressure CH4 pyrolysis by three-photon ionization via two-photon resonance of the 2S <-IS transition at 243 nm. An expected deuterium isotope shift of 22.4 cm-1 is observed during CD4 pyrolysis. Similar resonant multiphoton ionization signals of H atoms and D atoms are also observed from thermal dissociation of H2 and D2, respectively. The ionization signals are converted into absolute concentration via the H2 «= 2H equilibrium. The detection sensitivity is estimated to be -10-9-10-8 mole fraction at 1-atm pressure.