The paper reports on an experimental study of two turret-moored FPSO systems -an internal turret system and a bow-mounted external turret system. The set-up for the model tests is described in detail, including the characteristics of the systems tested and the environmental test conditions. Model test results are presented for motions and mooring line tensions for a range of long-crested and short-crested seas. The motion results for the two systems are used to identify general motion characteristics of turret-moored systems. Mooring line tension measurements are separated into high and low frequency components to enable line tension to be related to turret resonse. The important role played by turret position and wave spreading is highlighted. Consideration is given to how the maxima of the high and low frequency components of line tension combine to produce the maximum dynamic line tension.