Standard dual-energy CT uses two different x-ray energies to obtain energy-dependent tissue attenuation information to allow quantitative material decomposition. Combined use of dual-energy CT and PET may provide a more comprehensive characterization of disease states in cancer and many other diseases. However, integration of dual-energy CT with PET is not trivial, either requiring costly hardware upgrade or increasing radiation dose. This paper proposes a novel dual-energy CT imaging method that is enabled by the already-available PET data on PET/CT. Instead of using a second x-ray CT scan with a different energy, this method exploits time-of-flight PET image reconstruction to obtain a 511 keV gamma-ray attenuation image from PET emission data and combines the high-energy gamma-ray CT image with the low-energy x-ray CT of PET/CT to provide a pair of dual-energy CT images. We conducted a computer simulation to test the concept for material decomposition using air, soft tissue, fat and calcium. The simulations results indicate that this PET-enabled dual-energy CT method is promising for quantitative material decomposition, though future work is needed for noise supression. The proposed method can be readily implemented on time-of-flight PET/CT scanners to enable simultaneous PET and dual-energy CT for multiparametric imaging.