Nonspecular reflection effects of a bounded ultrasonic beam incident from a liquid onto an elastic structure have been the subject of a great deal of interest during last decades for material characterization [1–10]. It refers to phenomena where the reflected beam has an intensity profile different from that of the incident beam, including a lateral beam displacement, one or several minimum intensity area and a trailing field (Fig. 1). This phenomena occurs when the incident beam is phase-matched to one of the leaky waves supported by the structure. Numerous theoretical studies, based on the calculation of the reflection coefficient, have successfully explained the nonspecular reflection profile of a bounded beam incident at a critical angle [1, 3, 6–10]. In this paper, we present a mode theory for analyzing these nonspecular reflection effects. This approach, which gives a good physical insight, has been recently used to study the excitation of Lamb waves by the a bounded beam [11].