The synthesis of metal nanoclusters in a polymeric environment has been shown to yield nearly monodisperse particles, whose size is controlled by the strength of the polymer/metal interactions. Although the phenomenon is quite general, little is known regarding the mechanism by which the polymer controls nanocluster size. Previous studies of the kinetics of nanocluster growth in polymeric melts above the glass transition temperature (T-g) suggest that the nanoparticle size is set by the critical cluster size (nucleation stage) rather than the rate of metal precursor transport, namely, growth. In this paper, we examine the kinetics of iron oxide (Fe2O3) nanocluster formation below the glass temperature (T-g) in two polymer melts: polystyrene (PS) and poly(methyl methacrylate) (PMMA). We find that the morphology of the nanoclusters formed below T-g is highly sensitive to the system temperature and differs significantly from the morphology above T-g. However, the kinetics of cluster formation is exponential with time in both PS and PMMA, both above and below T-g. The glass transition does not significantly affect the rate constant in PS, thereby suggesting that the cluster formation mechanism is largely insensitive to the polymer state (glassy or melt). However, we find a significant difference in in the kinetics of PMMA above (where the rate constant increases exponentially with T) and below (where it is nearly constant) the, T-g.