Sol-gel processing of Ta(OC2H5)(5) to produce tantala highly reflective (HR) coating has been systematically studied in the absence of inorganic anions using an ethanoic acid catalyst in part using IR. In the presence of ethanoic acid, both hydrolysis and chelation co-exist, depending on the preparative parameters; most of the precursor was found to be converted into an amorphous chemically modified tantala sol and finally a gel, the crystallization of which did not start below 700 K. A ternary phase diagram was used to outline the regions of stability of the system. Two-layer and multi-layer tantala thin HR solid films (0.3 mu m thick with a refractive index of 1.7) can be prepared by spin-coating of such sols.