Iterative processing, involving sequential deformation and annealing, has been carried out on copper specimens with the aim of grain boundary engineering (GBE) them. The data have provided some interesting insights into the mechanisms of GBE. The results have demonstrated that development of a high proportion of Σ3s is beneficial to properties, as shown by improved strain-to-failure for the same strength. The proportion of Σ3s saturates at approximately 60% length fraction. Analysis of the data indicates that iterative processing is not always necessary for the development of beneficial properties, and it is further suggested that the condition of the starting specimen has a large influence on the subsequent microstructural development. The present, new data are also compared with previous research on copper where all five parameters of the grain boundary network population have been measured.