The propagation characteristics of a leaky surface acoustic wave on proton-exchanged 36 (degrees) Y-X LiTaO[inf 3] were experimentally investigated to evaluate the propagation loss. To measure the insertion loss, a pair of interdigital transducers was fabricated on a 36 (degrees) Y-X LiTaO[inf 3] substrate with the proton-exchanged layer, which was formed by immersion in phosphoric acid at 240 (degrees)C. The period of a pair of fingers ((lambda)) was 20 (mu)m. Each transducer was composed of eight finger pairs with an overlap length of 100(lambda). The distance of the free surface between two transducers was 250(lambda). The measured insertion loss at the center frequency decreased from 24.4 dB for the unexchanged sample to 16.8 dB for the 4-h proton-exchanged one, though the reflection coefficient of the transducer increased from 0.27 to 0.36. Therefore, the above result means that the propagation signal is improved at least 0.03 dB/(lambda) by the proton-exchange process. This phenomenon is probably due to the field confinement in the vicinity of the surface, which is caused by the formation of the proton-exchanged layer with a slow phase velocity as compared with an unexchanged substrate.