Structural and magnetization reversal studies have been carried out on single crystal Co2FeSi thin films grown on MgO (001) substrates. The films are highly L21 ordered after annealing above 500 °C. Magnetization reversal has been investigated by measurements of the activation volumes (Vact) within the films. This volume represents the unit of reversal in a magnetic material. Vact (∼4 × 103 nm3) has been found to be independent of the physical structure. Vact is found to correspond to an array of periodic misfit dislocations at the Co2FeSi/MgO interface. Such a small Vact potentially prevents coherent magnetization reversal as required for giant magnetoresistance or tunnel magnetoresistance devices.