A new nondestructive technique for the determination of the depth of the proton exchanged (PE) layer in LiNbO3 is reported. The techniques relies on the difference in phase of measured pyroelectric voltages, induced by a modulated laser beam incident on the exchanged and unexchanged regions. Measurements of PE layer thickness in a Z-cut LiNbO3 sample have been corroborated by prevalent, more elaborate, optical characterisation techniques. © 1993, The Institution of Electrical Engineers. All rights reserved.