Moving slugs of plastic pellets were investigated in-situ during low velocity pneumatic conveying in horizontal pipelines. Slug characteristics including the profile of pressure, pressure gradient, particle velocity, porosity, radial and wall shear stresses, aspect and behaviour were combined to obtain a complete picture of moving slugs. The objective was to gain unique knowledge on the physical mechanisms involved in slug formation, transport, and decay and the occurrence of pipe blockage. Slugs in both stable and unstable states were analysed. A strong correlation between particle velocity and wall stresses was found, which suggests that the stresses responsible for the high pressure loss characterising slug flow may result mostly from the transfer of particle impulses to the pipe wall. Most slugs were found to be denser at the rear where particle velocity was the highest, thus leading to slug shortening over time. This phenomenon was successfully modelled using both Newton's 2nd law and the ideal gas law and prediction of particle velocity showed good agreement with experimental values. In contrast, other slugs were found to extend due to the particles at the front moving faster than the particles at the rear. Pipe blockage was found to result from insufficient permeation of the slug by the conveying gas, indicating that sufficient material permeability is a condition for slug flow to occur. (C) 2014 Elsevier B.V. All rights reserved.