In this article the authors report on a new optical linewidth metrology method which uses measured changes in the resonant wavelength of fabricated ring resonator structures to establish the linewidth of waveguides. The technique has a precision of better than 0.5 nm for 500 nm linewidths and can be used for process control although it cannot be used with arbitrary optical structures. To demonstrate the technique they fabricated ring resonator structures on silicon on insulator substrates several times over a period of 70 days, using electron beam lithography with hydrogen silsesquioxane (HSQ) negative tone resist followed by dry etching. They show that waveguide width variations of less than 0.5 nm can consistently be achieved when the HSQ is diluted immediately prior to use but highly irreproducible results are soon obtained if a single dilution is prepared and used for an extended period.