Deposition of large-area electrochromic films is essential for preparation of commercially viable electrochromic windows for architectural applications. This paper describes the preparation of commercially practical sol-gel precursors for large-area film deposition, and the colouration behaviour of these coatings. The preparation of the sol-gel precursors is an adaptation of a standard method of preparation from metal halides, It has been developed to minimise contamination and modification of the precursors which often result if HCl is removed by NH3 as an NH4Cl precipitate. This contamination and modification lead to irreversible behaviour in the final films. Details of the performance of these films is given, including the electrical and optical performance. The results show excellent reversibility of colouration for the films prepared with the precursors made by the new method, compared with relatively poor performance with precursors made by the standard method from metal chlorides and NH3.