The evaporation rates of bulk liquid and thin films of an alcohol-derivatized perfluoropolyether have been studied experimentally and computationally. We find that, even in the case of narrowly fractionated material, the time dependence of the evaporation rate in both the bulk and thin-film cases is dominated by the residual polydispersity of the samples, The bulk and thin-film evaporation can be described very well by a model that incorporates the molecular weight distribution, molecular-weight-dependent Arrhenius parameters of evaporation, and Raoult's law of vapor pressures. Minor corrections to the model that account for surface interactions are necessary in the case of thin film evaporation.