The results of a molecular dynamics simulation study of water acetonitrile mixtures over the temperature range 297 - 415 K are reported. The emphasis is on the microheterogeneous structure present in these mixtures for compositions ranging from 0.1 - 0.9 mole fraction of water. The characterization of the microstructure is presented in terms of site-site pair correlation functions and in terms of the size and shape of water clusters and acetonitrile clusters.