Provided are methods of forming optical waveguides. The methods involve: (a) forming over a substrate a layer of a photodefinable composition; (b) exposing a portion of the layer to actinic radiation; (c) developing the exposed layer to form a waveguide core structure; and (d) heating the waveguide core structure to a temperature and for a time effective to reflow the structure such that it becomes at least partially rounded in cross-section. Also provided are optical waveguides formed from the described methods and electronic devices including one or more of the optical waveguides.