We have used picosecond ultrasonic techniques as a nondestructive detection tool to characterize Mo/Si multilayer reflectors for EUV. The lowest two localized acoustic-phonon surface modes were simultaneously observed in our samples with various d and (Gamma) values. The vibration frequencies of these surface modes depend both on d and (Gamma) , and can be used to extract these two parameters. We mapped the thickness profile of a linearly graded Mo/Si multilayer sample with 2 percent d-variation from the two edges towards the center. The dependence of the vibration frequencies on (Gamma) was also studied theoretically and experimentally, and was found to be separable from the dependence on d.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.