As a result of a number of high altitude submillimetre atmospheric observations using both the Sun1-4 and the sky5,6 as a source, there is interest concerning the origin of a number of weak absorption features that have appeared in these spectra. In particular a result obtained at an altitude of 12.6 km (ref. 4) has given tentative assignments to a number of the features observed. We present here laboratory results aimed at explaining certain features in the region 19 to 31 cm-1 in terms of isotopic modifications of water vapour (HDO and H2O18) (ref. 7) and the rotational transitions of the lowest vibrationally excited state of H2O16 (v1, v2, v3 Combining double low line 0, 1, 0, centred at 1,595 cm-1 (ref. 8) Transitions resulting from these species apparently have not been considered in analysing atmospheric results. Also we present a result which has been referred to in both refs. 2 and 4, namely, an atmospheric absorption spectrum in the region 40-54 cm-1, again with the purpose of identifying any weak features attributable to both isotopic species of water vapour, and the dimer of the water molecule2,3,9. © 1969 Nature Publishing Group.