X-ray diffraction measurements of the structure of liquid potassium-caesium alloys have been performed throughout the composition range between 40° and 100°C. Special attention has been paid to the small-angle region in order to study the thermodynamic behaviour. More particularly, the SCC(0) has been determined as a function of the composition and has been related to measurements of the vapour pressure and the heat of mixing. The measurements at higher q have been compared with two theoretical models. The first one is the well-known solution for hard-spheres of the Percus-Yevick equation, the second one is derived from it by introducing an effective softness