A molecular-dynamics study has been made of the influence of solute content on low-energy displacement cascade processes in copper containing up to 15 at. % Au in solid solution. This alloy system was chosen to illustrate the effects of heavy, oversized solute atoms in a fcc matrix. Cascades of up to 2ke Vin energy were considered. The presence of the solute was found to have a significant impact on all phases of cascade development, but not on the final defect number. The ballistic phase is increased in duration, and the number of temporarily displaced atoms is increased by the solute, and the thermal spike that follows is also increased in intensity and duration. As a consequence, the atomic mixing associated with these two phases was highest in the Cu–15% Au alloy that contained the most solute. However, as a result of self-healing in the thermal spike, the final number of Frenkel defects is independent of alloy composition. To test the influence of solute atomic mass, simulations were carried out on copper containing solute atoms with the same size as Au, but a lighter mass than copper. The alloys with this fictitious element did not show the ballistic-and thermal-spike effects exhibited by those containing "normal" gold. The results are discussed in terms of the likely roles of solutes in damage evolution in alloys under cascade-producing irradiation.