This paper presents the results of an experimental investigation of the effects of mixture velocity and overall concentration on local concentration of coarse particles conveyed by water in a smooth horizontal pipe. A fraction of basalt pebbles with a narrow particle size distribution and particle sizes ranging from 8-16mm was used as the solid phase. The mixture flow behaviour and concentration distribution were investigated in an experimental pipeline loop of inner diameter 100mm. In a pipe cross-section, the local particle concentration distribution (chord-averaged concentration profiles and concentration maps) was measured using a -ray densitometer and compared with the model proposed by Gillies and Shook. This study confirms the existence of significant stratification of the studied mixtures and stresses the importance of saltation movement in the transport of coarse particles in a pipe. The difference between measured and predicted chord-averaged concentration profiles decreased with increasing mixture velocity and mean concentration.