A numerical analysis is performed for the electroosmotic flow of two miscible electrolyte streams in a T-shaped microchannel. Rich dynamic features are revealed concerning the onset and growth of the electrohydrodynamic instability. It is shown the molecular diffusion has dual role in the onset and development of instability, and the system is unstable to high electric fields even without any external disturbances. The molecular diffusion process is shown to play a key role in making the wave pattern looks regular and deterministic. Detailed mechanisms in the latter stage of instability growth and formation of spike-like wave patterns are also explained. (c) 2006 Elsevier Ltd. All rights reserved.