The results of fluorescence experiments carried out on two ethylene-propylene copolymers (EP) labeled with pyrene, naphthalene, or both provide the first experimental evidence of intrachain formation of crystalline microdomains in solution. Differential scanning calorimetry (DSC) experiments performed on 50 g/L EP solutions in toluene demonstrated that one EP copolymer was crystalline (EP-CR) while the other was amorphous (EP-NCR). The intrinsic viscosity of EP-CR and EP-Nu CR in toluene was measured as a function of temperature. EP-CR was the only sample to exhibit a decrease in intrinsic viscosity around 0 degrees C, suggesting a decrease of the hydrodynamic volume of the polymer coil as expected if EP-CR forms microcrystals in solution. The light scattering intensity of 2 g/L solutions of EP-CR and EP-Nu CR was also measured as a function of temperature. The EP-CR solutions showed an increase in light scattering intensity at temperatures below 0 degrees C. The same observations were made with the fluorescently labeled EPs using DSC, intrinsic viscosity, and light scattering measurements, demonstrating that fluorescently labeling the polymers did not affect their solution properties. However, the high sensitivity of fluorescence enabled us to perform experiments at polymer concentrations as low as 20 mg/L. Experiments were carried out using fluorescence resonance energy transfer and pyrene excimer formation that established the absence of intermolecular association at this polymer concentration. These fluorescence experiments indicate that EP-CR undergoes a transition at 0 degrees C in toluene consistent with a decrease of the hydrodynamic volume of the polymer coil. These experiments constitute the first evidence that crystallization of semicrystalline EPs in solution can occur intramolecularly.