In this study transparent films of poly(ethylene terephthalate)-co-(ethylene 2,4-furandicarboxylate) (PET-co-2,4-PEF) copolymers were developed for the first time, exploring the ability of 2,4-furandicarboxylic acid (2,4-FDCA) to impart excellent optical properties to the polymers thereof. The properties were fine-tuned by feeding this furan monomer content. For example, specifically up to 15 mol% the copolyesters were semi-crystalline in nature, and they were amorphous materials when exceeding the limits of 50 mol% and above. The amorphous polymers had excellent thermal stability over 400 °C, in combination with high transmittance at the wavelength of 555 nm.