ABSTRACT： This is a study of the flow properties of a range of magnetic toners produced from a common base in relation to three physical variables – the milling method, the type and amount of flow additive and to a limited extent, particle size. Two methods of milling were used for sample preparation – air jet milling and mechanical milling. The flow additives were either HMDS or PDMS silicas. The study used the FT3 powder Rheometer (Freeman Technology) to determine the rheology of powder samples in terms of the energy required to cause a powder to flow. The dependence upon environmental factors commonly imposed on powders such as flow rate, the level of aeration and the amount of consolidation are determined for each of the materials. The propensity to retain air is also investigated and quantified. The results clearly indicate the complex nature of toners and how their flow properties are affected by a large number of variables, of which one of the most significant is the effect of aeration and de-aeration representing the transition from fluidisation to consolidation.