Using differential scanning calorimetry (DSC) and X-ray analysis it is shown that binary systems of perfluorinated alkanes form solid solutions or exhibit eutectic phase behaviour, depending on their difference in chain length. A simple model, which successfully correlated experimental results for this aspect of phase behaviour of binary n-alkane systems, is demonstrated to quantitatively describe the behaviour of perfluorinated alkanes as well. From the model parameters and experimental observations, it appears that, at equal number of carbon atoms, binary perfluorinated alkane systems allow for a larger chain difference than their hydrogenated analogues, before eutectic phase behaviour sets in.