An infrared spectroscopic method for measuring the percentage of the crystalline material in the α-modification is developed for poly[3,3-bis(chloromethyl)oxacyclobutane] (Penton). Good agreement with an X-ray method is found. This spectroscopic analysis is used to study the limiting conversion of β- to α-crystallinity during the heat-induced polymorphic transformation in Penton. The observed behavior is interpreted using the independent domain theory coupled with nucleation concepts.