In biomedicine, polymer blends are frequently applied in wound dressing design or drug delivery. Within these applications, poly(2-alkyl/aryl-2-oxazoline)s (PAOx) are emerging as a popular matrix due to excellent biocompatibility and miscibility with other polymers. However, as much is known of PAOx miscibility with other biocompatible polymer systems, so little is known of the miscibility within the PAOx class. We show the remarkable phase separation of two important, structurally alike, amorphous PAOx, i.e., poly(2-ethyl2-oxazoline) and poly(2-n-propyl-2-oxazoline), that occurs when the polymers' number-average molar mass exceeds 10 kg.mol(-1). The (im)miscibility as a function of average molar mass is experimentally investigated by thermal analysis, theoretically underpinned by the Flory-Huggins lattice theory, and visualized by fluorescence microscopy in both films and nanofibers, the latter being a high-potential support material in biomedicine. These results provide important knowledge on PAOx (im)miscibility which has a crucial impact on the behavior of the many final end products they are investigated for.