Physical mechanisms responsible for the suspension of solid particles in semi-batch bubble column slurry reactors in bubble flow are identified, thus allowing the classification of solids into four states: free, engaged, wake-entrained and captured particles. A theory is then presented to predict the axial distribution of solids when wake-entrained and captured particles are absent. Reasonable agreement is found between the theory and experimental data obtained in this work and those reported by otherinvestigators.