Impurity-helium condensates containing krypton atoms and also atoms and molecules of hydrogen isotopes have been studied via an electron spin resonance (ESR) technique. Analysis of the ESR spectra shows that most of the H and D atoms reside in molecular layers (H(2) or D(2)) formed on the surfaces of Kr nanoclusters. The thickness of the molecular films was found to determine the rates of recombination of the atoms into molecules, with atoms in the thinner films recombining much more slowly. Very large average concentrations were obtained for H atoms (10(19) cm(-3)) and D atoms (3a <...10(19) cm(-3)) in these experiments.