In this paper the asymmetric water sloshing in a partially filled circular cylindrical container subjected to horizontal harmonic excitations is investigated. The understanding of this phenomenon is fundamental for the design of propellant management devices; moreover, the dynamic response of the moving liquid inside the tank can dramatically influence the trajectory of vehicles. The asymmetric sloshing is controlled by the liquid depth, forcing frequency and amplitude. Above all, the behavior of the free liquid surface is of major interest. The analytical and equivalent mechanical models of the simplified undamped problem are here introduced. Fluorescent Particle Image Velocimetry is used to investigate the velocity field in the liquid phase. Phase locked PIV with a selected phase of the external excitation is used in order to provide statistical data. An image processing algorithm is developed to detect the free surface and to properly define the fluid field of interests. The commercial algorithms of PIV and the combined PIV+ PTV are compared. Selected examples of information that can be extracted from the PIV phase locked results for the sloshing phenomenon are presented.