AbstractIt was found that cellulose diacetate (the degree of substitution = 2.46) dissolved in 2‐butanone but that these solutions presented cloud points in both regions of the lower and upper critical solution temperatures (LCST and UCST). The cloud points were measured for one unfractionated and three well‐fractionated samples, from which the LCST and UCST were determined. The CST data were plotted according to Shultz and Flory, to yield the results of θu=310Kand θL= 433K. Here, θ is the theta point and the subscripts U and L refer to the UCST and LCST respectively. The present CST data were first compared with the prediction of a corresponding states theory by Patterson. When the Cowie‐McEwen semi‐empirical rule was employed to estimate the solvent characteristic temperature, a quantative agreement with the observed LCSTs was obtained, while the predicted USTs were too low in comparison with the observed ones. Secondly, an analysis according to Eichinger's theory offered information showing that the thermodynamic parameters of the present system were less temperature dependent. Differences between the present and the ordinary systems to which the Cowie‐McEwen method was applicable were enumerated, each of the being examined from several points of view. A consideration that the 2‐butanone molecules were solvated onto the cellulose diacetate molecules was helpful to explain the solubility behaviour of the p