Abstract The vibrational spectra of H 2, D 2 and HD trapped in solid krypton were examined as a function of concentration and sample annealing. Two kinds of aggregate were identified, one before, the other after annealing. Before annealing, three lines successively appear in each of the Q( J) domains for hydrogen/Kr molar ratios increasing from 0.01 to 0.04. The first line has been assigned previously to monomers; the two others, 1±0.3 and 2± 0.5 cm −1 blue-shifted with respect to the first one, are respectively assigned to dimers and trimers. After annealing above 38 K, totally different spectra are observed, with properties comparable to those reported under the same conditions in argon matrices and also to those of the crystalline phase. The results suggest that dihydrogen is trapped after annealing in microcrystals. Such a phase separation can be explained by the low solubility and the mobility of dihydrogen.