State resolved densities of high rovibrationally excited hydrogen isotopologues H 2 , HD, and D 2 in the electronic ground state have been measured in a supersonically expanding plasma jet. The obtained state distributions differ substantially from thermal equilibrium. Moreover, the distributions are not the same for H 2 , HD, and D 2 indicating different formation and relaxation rates for each isotopologue. Mechanisms for this deviation from a Boltzmann distribution are given and compared to hydrogen reactions in other environments. The difference between the measured highest occupied rovibrational states in H 2 , HD, and D 2 is ascribed to an isotope effect in the dissociation process.

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