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tokens: environments. Our results will aid in the preparation of multicomponent ices containing acetaldehyde by allowing for calibrations and determinations of that compound in an ice sample. We also envision applications of our data to the measurement of reaction yields in experiments that might produce acetaldehyde in polar ices. Our IR intensities also can be used to estimate band strengths in spectral regions beyond the mid-IR used in the present work. See Gerakines et al. ( 2005 ) for an example. Astrochemical results based on IR band strengths published before those presented here can now be updated with our new measurements. For example, and as already described, the reference band strength of Schutte et al. ( 1999 ) for amorphous acetaldehyde's CH 3 deformation at 1350 cm −1 is A ′ = 1.5 × 10 −18 cm molecule −1 , which now is seen to be in error by nearly 80 per cent. Molecular abundances or column densities based on that older A ′ will be in error by that much, although published relative band strengths might not change substantially. Finally, we again point out that few aldehydes have been examined with the methods used here. As an example, propionaldehyde (propanal) is an interstellar molecule, and at least two laboratory papers recently have been published involving it (Jonusas, Guillemin Krim 2017 ; Qasim et al. 2019 ), but no band strength measurements are available for the solid compound. The situation is no better for H 2 CO (formaldehyde). Many laboratory papers have been published involving H 2 CO, but only estimates and indirect measurements are available for its IR intensities. Unfortunately, their accuracy is unknown. 5 SUMMARY AND CONCLUSIONS Here we have reported the first direct measurements of band intensities of solid acetaldehyde, removing the need to rely on either calculated gas-phase results or values that cannot be traced to their original source. We also have presented mid-IR spectra of the two crystalline polymorphs previously identified by neutron-diffraction studies, one stable under our conditions and one metastable. The IR spectrum of H 13 C(O) 13 CH 3 has been recorded and used to identify ketene as a radiolytic decomposition product of solid acetaldehyde for the first time. APPENDIX A The procedure used to determine absorption coefficients ( α ′) and band strengths ( A ′) is the same as in our earlier papers, and is essentially the method of Hollenberg Dows ( 1961 ). Briefly, IR spectra were recorded of ice samples having a range of thicknesses. The absorbance at each peak of interest was measured and bands of interest were integrated. Multiplication of each absorbance or band area by ln(10) ≈ 2.303 was then done to convert from a common to a natural logarithmic scale. Representative plots of the results are shown in Fig. A1 . The slope of the upper graph is α ′ and the slope of the lower one is ( A ′ ρN A / M ), where M = molar mass = 44.05 g mol −1 , N A = 6.022 × 10 23 molecules mol −1 , and density ( ρ