The infrared spectra of dilute solutions of acetone in a range of protic and aprotic solvents have been measured in the C—O stretch region. Band maxima are shown to correlate linearly with ¹³C n.m.r. shifts for the carbonyl carbon and, after suitable corrections, with solvent acceptor numbers. In mixed solvent systems involving protic and aprotic solvents, bands are gained and lost, indicative of the presence of discrete solvates. We infer that acetone in methanol is ca. 90% monohydrogen-bonded and ca. 10% non-hydrogen bonded. As basic aprotic cosolvents are added, the former species is lost and the latter gained, the efficiencies of these changes being proportional to the donor numbers (base strengths) of the cosolvents (cyanomethane, dimethyl sulphoxide, hexamethylphosphoramide and triethylamine).

For systems involving water + basic aprotic cosolvents, three distinct species were detected. We suggest that the species present in pure water is acetone with two hydrogen bonds to water, that dominating in the middle range, with ν_max close to that for methanolic solutions, has one hydrogen bond and the third species forms no such bonds. The overall tendency to dehydrate is again proportional to the donor number of the cosolvent except in the water-rich region, where the solvation number of the cosolvent is thought to be the overriding factor.

Small shifts in the band maxima (ν_max) for these discrete species with change in solvent composition are discussed in terms of secondary solvation and specific interactions between acetone and aprotic solvent molecules. The latter is particularly marked for methanol + dimethyl sulphoxide systems.

Attempts are made to justify our assignments of the species detected in protic solvents to specifically hydrogen-bonded units. In particular, our suggestion that acetone is completely present as doubly hydrogen-bonded units in dilute aqueous solutions despite the evidence that the resulting hydrogen bonds are weaker than the average water–water bonds is defended.