This study investigates crystal changes in Ziegler/Natta-based ethylene-propylene random copolymers subjected to room-temperature annealing. These copolymers, which are being considered as a new class of thermoplastic elastomers, exhibit slow crystallization kinetics. This slow crystallization causes subsequent changes in mechanical behavior that take place over days and even weeks. An initial study consisting of four copolymer compositions is conducted. Crystallization is monitored via differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) as a function of sample age and composition. For these copolymers WAXD reveals a change in crystal structure from orthorhombic to hexagonal well below the triple point of polyethylene at room temperature and pressure. Mechanical behavior is also monitored in a similar fashion. Evolution of modulus, yield stress, yield strain, and rupture energy are correlated with changes in crystal structure observed through DSC and WAXD.