In this study we propose a model that describes the diffusion of copper, hydrogen, and titanium through the liquid/solid interface in potassium lithium tantalate niobate crystals. Copper and hydrogen ions may occupy potassium/lithium vacant sites. Hydrogen ions are also able to stay at tantalate/niobate vacant sites in the presence of titanium. This model shows that the hydrogen concentration can be dramatically reduced in KLTN:Cu,X,Ycrystals (where X, Yare any first row transition metals but not Ti or Cu), and is in agreement with experimental results. © 1997 American Institute of Physics.