We report on a series of polyion complexes from mixtures of poly(ethylene oxide)-block -poly(N,NN,N-diethylaminoethylmethacrylate) (PEO–PDEAMA) and poly(ethylene oxide)-block-poly(aspartic acid) (PEO–PAsp). As expected, the micelle size, polydispersity and stability are dependant on the relative and absolute lengths of the polyelectrolyte chains. However, we also demonstrate that whilst the length of the charged polyelectrolyte blocks is important, the length of the PEO chains is an equally relevant variable in determining both the size and stability of the final micelles as well as the degree of charge neutralisation at which micellisation occurs. We also show that the kinetics of formation can result in very different stability of the final micelles.