Betulin, an abundant triterpene, can be extracted from birch bark and can be used as a renewable monomer in the synthesis of microporous polyesters. Cross-linked networks and hyperbranched polymers are accessible by an A2 + B3 reaction, with betulin being the A2 monomer and B3 being a trifunctional acid chloride. Reaction of betulin with a diacid dichloride results in linear, soluble polyesters. The present communication proves that the polyreaction follows the classic schemes of polycondensation reactions. The resulting polymers are analyzed with regard to their micro-porosity by gas sorption, NMR spectroscopy, and X-ray scattering methods. The polymers feature intrinsic microporosity, having ultrasmall pores, which makes them candidates for gas separation membranes, e.g., for the separation of CO2 from N2.