Particle image velocimetry (PIV) has been applied to the study of acoustic flow of liquid in a standing wave tube. Even though liquid compressibility is very small, the liquid must be treated as compressible in this case. With the finite compressibility of liquid in mind, a series of different standing wave modes can be formed by pressure waves emanated at specific driving frequencies from a bimorph piezo disk at the end of the tube. In this paper, the first three natural standing wave modes were visualized using 1 μm diameter fluorescent microspheres seeded in the liquid. The variation of the flow field in the acoustic boundary layer near the wall was measured using PIV. Water was first used as a working fluid. Experiments were then carried out with a glycerol–water mixture (50%–50% by volume) to examine the effect of viscosity change on the wave propagation and flow structure inside the tube. The experimental results are compared with theoretical model predictions.