The present report is concerned with the evolution of boundary layers during runup of solitary waves on a beach in a wave tank of depth 0.2m. It comprises both theory and high resolution PIV measurements of velocity profiles. A linear stability analysis of the boundary layer for solitary waves running up a sloping beach is performed by means of the Orr-Sommerfeld equation. Due to the increased retardation phase during runup, the amplification of disturbances in the boundary layer is increased as compared to that of solitary waves traveling on constant depth. On the basis of these results, we reexamine the experimental results by Pedersen et al.  and find some experimental evidence for Tollmien-Schlichting waves destabilizing the flow.