The effects of wheat bran particle size on aeration dynamics during high speed mixing of bread dough and on the expansion capacity of doughs were investigated. Commercial wheat bran was milled to different particle size ranges and substituted for flour at a level of 15% in a bread dough formulation. Mixing doughs at different headspace pressures allowed the gas-free dough density and gas content at atmospheric pressure to be determined. Addition of wheat bran decreased the gas-free dough density (in part due to extra water in the dough formulation) and increased the gas content of doughs. Experiments with an inert particulate analogue, glass ballotini, confirmed that at least part of the increase in aeration was due to the particulate nature of the bran. Larger bran particles gave slightly lower gas-free densities and higher gas contents. Pressure step-change experiments allowed the rates of entrainment and disentrainment of gas during mixing to be quantified using a population balance model. The presence of bran at 15% increased the rates of entrainment and disentrainment, and hence the rate of turnover of gas during mixing, and changed the balance in favour of entrainment, hence giving a higher steady state gas content. Larger bran particles increased the rate of turnover of gas and gave a higher steady state void fraction of gas than smaller particles. Bran decreased the expansion capacity of doughs as measured by the dynamic dough density test, with smaller particles resulting in lower expansion than large particles.