An experimental study dedicated to the characterization of the settleability of solids produced in immersed fixed media biofilm reactors has been carried out. The influence of operating temperature (0.1 to 16 °C) and surface organic loading rate (OLR) (0.4 to 10 g of soluble carbonaceous BOD5 per m2 of media per day) on settleable solids quantities, particle size distributions (PSD) as well as flocs morphology was evaluated. Results have shown that the OLR has no statistically significant influence on the settleability of the suspended solids. However, the operating temperature was identified as a factor that significantly influences the settling potential. The highest operating temperatures (14–16 °C) were related to the worst settling performances. On the other hand, the best settling performances were observed at intermediate operating temperatures (around 10 °C). The latter conditions were also associated with the largest fractions of large particles (>100 μm) in the effluent. Differences in PSD were found to be well correlated with settling performances. Part of the performance results variability which cannot be explained by differences in PSD can potentially be attributed to differences in flocs morphology (compactness).