Research has been carried out to determine the effect of filler coatings on the processing properties of medium density polyethylene (MDPE) modified by an ultrafine grade of flame-retardant magnesium hydroxide (Mg(OH)(2)) filler. Selected filler coatings were acid-group terminated and were of varying aliphatic chain length. The filler dry-coating process has been optimized by characterizing the reaction between Mg(OH)(2) filler surface and the acid group, using spectroscopic techniques including Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). Using immersion calorimetry, the interactions between the fillers and the polyolefin matrix were shown to decrease on the addition of a fatty-acid coating. Compounding torque and specific energy data relate to filler dispersion; qualitative analysis has demonstrated how the coatings provide a reduction in both size and number of particle agglomerates. MDPE compound processability was assessed by capillary rheometry; wall slip was evident in compounds containing uncoated Mg(OH)(2) filler. Consequently, the development of molecular orientation of the polymer during injection mold filling, quantified by a reversion analysis, is modified by the effects of filler coating chain length and addition level, an effect that has important implications to link the mechanical properties of MDPE-Mg(OH)(2) composites to processing history.