When flow in diverging pipe separates from the pipe walls, there occurs an increasing loss of energy and at the same time the flow begins to vibrate, being accompanied by other trouble. In this paper we describe the results of experiments on flow in a 20° conical diffuser. In the initial state of separation a reverse flow takes place near the pipe walls, the main flow going straight ahead in the form of a jet entirely uninfluenced by the diverging walls. The points of separation of the flow can be determined by the sudden increase in boundary layer thickness (displacement thickness). In this initial state of separation, the value of efficiency of pressure recovery is slightly lower than its maximum. The efficiency increases somewhat with increasing Reynolds number and with decrease in boundary layer thickness at the pipe entrance. There is considerable variation in pressure which is the order of 5-10% and the main flow swirls around within the pipe. A comparison of the separation points calculated by the theory of one of the present authors with the experimental results shows good agreement.