Reactions of deuterated analogues of H+3 ions with HD form a deuteration reaction-chain H3+ H2D+ D3+ suggested by Philips and Vastel [1] to justify the expectation of high [D]/[H] ratio in prestellar cores. The right-to-left direction in the chain ("de-deuteration") is highly improbable in the conditions of the interstellar medium prior to star formation (T < 10 K) if the reactant H2 are in para nuclear spin states and resulting deuterated ions are in their ground levels, because of the endothermicity DH ~ 100 K.