Changes take place in the o.r.d. spectra of optically active arylazoalkanes on exposure to light : these are attributed to cis–trans–isomerisation about the NN bond. The isolation of the cis-form of 3β-phenylazo-5α-cholestane is described, and its spectroscopic properties are discussed. Light-catalysed changes in the molecular rotations and o.r.d. curves of arylazo-sugars which have been observed previously are now attributed to cis-trans-isomerism rather than to conformational chair–boat equilibria as suggested earlier. In support of this hypothesis, two aliphatic vicinal arylazo-alcohols have been prepared and found to exhibit light-catalysed isomerism, qualitatively identical to that of arylazoalkanes lacking the hydroxy-group.