Monodisperse ZnO colloidal nanocrystal clusters (CNCs) are synthesized through a rapid microwave-polyol process. The size of the clusters is tunable from 57 to 274 nm by varying the amount of Zn-complex precursors. The combination of unique secondary nanostructures, inter-nanocrystallites, and large internal surfaces make these ZnO CNCs ideal candidates for applications such as humidity sensing.