Decay patterns of atomic hydrogen trapped in argon and krypton matrices are followed by electron paramagnetic resonance (EPR). Hydrogen atoms are generated by uv-photolysis of HBr and HCl precursor molecules. The EPR signals due to interstitially trapped hydrogen atoms in octahedral sites disappear near 16 and 24 K in Ar and Kr, respectively. Substitutionally trapped H atoms are thermally stable up to evaporation temperature of the solids. The fate of thermally released H atoms in Ar is exclusively due to geminate recombination of the parent molecule. The observed kinetics is well fitted with double exponential decay. The kinetic behavior reflects short-range dissociation and recombination dynamics in Ar. In the Kr matrix, a change from first-order to second-order kinetics is observed at higher concentrations as formation of molecular hydrogen becomes important. From bimolecular decay kinetics, a diffusion constant of 4 X 10(-15) cm(2) s(-1) is deduced for H-atom diffusion in Kr at 26.9 K. The obtained activation energies, 6-7 kJ/mol in Ar and 9-14 kJ/mol in Kr, are measures of thermally activated cage dynamics and show only weak dependence on the hydrogen isotope.