The velocity of ultrasound was measured in molten Au-Co alloys containing 0, 10, 20, 27, 30, 40, 50, 60, 70, 80, 90, and 100 at.% Co within the temperature range from 995°C to 1700°C. The temperature coefficient of the velocity versus concentration plot shows a positive deviation from ideal behaviour. The adiabatic compressibility was calculated from the velocity and is discussed. From it the partial structure factors aij(o) and Ski(O) for zero momentum transfer were obtained. According to these results, the melts of the Au-Co system show a weak tendency to segregation, i.e. they show concentration fluctuations. Between zero and 60 at.% Co these concentration fluctuations correspond to Au-enriched regions and Co-enriched regions, these being present to the same extent. Between 60 and 100 at.% cobalt the influence of Co-enriched regions is greater than that of the Au-enriched regions.