A series of bulk proton‐exchanged lithium niobate waveguides has been prepared by using concentrated sulfuric acid as the proton source and varying the time and temperature of the exchange process. The optical and physical properties of the proton‐exchanged lithium niobate layer have been measured and found to be essentially identical to proton‐exchanged layers obtained by other workers using the benzoic acid process. Sulfuric acid offers a number of advantages over benzoic acid as a proton source, including a wider temperature liquid range and a greater ease of handling in a clean‐room environment.