Closed-form theoretical expressions are derived for the integrated absorption coefficient of various zerophonon single and double transitions in heteronuclear isotopic variants of hydrogen in condensed phase. Theoretical analysis predicts a different kind of double transition in the spectra of solid HD, HT, and DT, where the rotational energy of one molecule changes by three quanta and the rotational energy of another molecule simultaneously changes by at least three quanta. The expressions for double transitions given in this paper may, for example, be applied to double transitions involving para-H₂–HD or ortho-D₂–HD pairs.