Experiments are performed in order to study the effect of hydrate formation on heat transfer in pipes. The hydrate formation in the horizonal flow of the gaseous refrigerant R134a and water is examined in a transparent test section at low pressure. The experiments conducted have shown that the formation of hydrates starts on the pipe wall. The whole perimeter of the pipe wall is covered by hydrates and a hydrate layer is achieved. The thickness,of the hydrate layer can be related to the measured heat transfer resistances and measured pressure losses. The thickness of the hydrate layer can be calculated analytically, if a first-order logarithmic function of the radial coordinate is assumed for the temperature field within the hydrate