The potassium of KTiOPO4has been exchanged for protons or ammonium ions to form optical waveguides. Proton exchange is relatively fast, does not give depth profiles expected for diffusive processes, and distinct potassium‐depleted phases are formed. The kinetics for ammonium exchange are relatively slow and diffusion limited, similar to those for Rb and Tl exchange processes. Increases in surface refractive index of up to 0.053 for proton exchange and 0.046 for ammonium exchange have been observed, with higher values for mixed NH4/H and Rb/H exchanges.