High temperature magnetization studies (up to 650 degrees C) of the chemically synthesized nanocrystalline Zn0.9Co0.1O samples are reported in this paper. A systematic study of sequentially sintered sample has been carried out to correlate the observed changes in magnetic behavior vis a vis very small changes in the unit-cell dimensions, evolution of cobalt oxide phases, and absence of Co clusters. The Curie temperature of cobalt substituted ZnO is determined to be similar to 495 degrees C. A plausible explanation of the observed room temperature ferromagnetism is presented in terms of bound magnetic polarons model. (c) 2007 American Institute of Physics.