Pecan shells are the byproduct of the processing of pecans for human consumption. The potential use of these shells for bioenergy and other value-added applications requires information on the physical and flow properties. These properties are needed for the design of storage, processing and handling equipment. The effects of particle size and moisture content on bulk density, particle density, tap density, compressibility, angle of repose, cohesion, angle of internal friction and flowability were quantified in this study. Equilibrium moisture relations for the shells were also determined at temperatures ranging from 10 to 50 degrees C. The densities (bulk, particle and tap) significantly decreased and compressibility significantly increased with increases in particle size and moisture content. Increasing the moisture content reduced its flowability from free flowing (flow index of 15.77) at a moisture content of 4.2% (w.b.) to cohesive (flow index 3.14) at a moisture content of 24.6% (w.b.). The cohesive strength of pecan shells also increased with particle size (0.58 to 1.14 kPa) and with moisture content (0.36 to 1.69 kPa). An average angle of internal friction of 41.28 degrees was obtained and was not significantly affected by moisture content and particle size. The angles of repose of pecan shells varied from 33.1 degrees to 46.3 degrees. The equilibrium moisture relation curves for the shells were sigmoidal in shape and showed that pecan shells should be stored at moisture contents of less than 18.03% (w.b.) to minimize microbial activity. The equilibrium moisture relation curves were best predicted by the modified Henderson equation. (C) 2011 Elsevier B.V. All rights reserved.