On the hypothesis that a mathematical relationship exists between synthesis conditions and hematite nanoparticle (NP) sizes, an empirical model was developed by synthesizing hematite NPs at different temperatures and hydrolysis times, and then correlating them with the obtained NP sizes. We found that the hematite NP sizes can be described by a simple relationship, d(P) = (0.49 x T - 26.4) X t(0.33). The predictive capabilities of the model were validated by synthesizing NPs at randomly selected experimental conditions and comparing them to the model predictions. The experimental findings suggested that the model may be limited to predicting NP sizes smaller than 50 nm because of a possible shift from a slow, hydrolysis-driven growth mechanism to a mechanism characterized by rapid aggregation of the existing NPs. (C) 2012 Elsevier B.V. All rights reserved.