Abstract. The coercivity and energy losses in superparamagnetic magnetite and FePt nanoparticle composites subjected to an external, alternating magnetic field have been calculated as a function of the mean particle-size and packing density. The effect of interactions has been investigated by fitting the Sharrock law to the coercivity results as a function of the field cycle frequency of the magnetic field. This fitting leads to effective parameters for the anisotropy field H ef f K and β ef f = KV /k B T , which are themselves dependent on the interaction strength. The increase or decrease of the coercivity with interactions depends upon the relative change of H ef f K and β ef f , thus demonstrating the complex effect that interactions have in these nanoparticle composites. The interparticle interactions have a non-trivial effect on the energy loss per cycle. The energy loss is reduced for systems with larger particles since the reduction in coercivity together with a corresponding reduction in the remanence dominates. For small particle sizes, the energy loss is increased. The primary mechanism here seems to be an enhancement of the energy barrier due to interactions, which changes the nature of the particles from superparamagnetic to being thermally stable.