Abstract: A non-destructive method and apparatus for determining the thickness of individual layers of different materials in a multilayer structure particularly useful in analyzing the layers in a multilayer plastic fuel tank, utilizes a constant and relatively high frequency ultrasonic pulse transmitted into the multilayer structure and records the time when interference pulses are received to determine the thickness of the individual layers in the multilayer structure. The interference pulses are generated at the interface between adjacent layers of sufficiently different materials and thus, the time between interference pulses is a function of the amount of time needed for the ultrasonic pulse to pass through a layer of material.