The theological properties of surface treated and untreated CaCO3-high-density polyethylene composites were studied and related to particle-particle and particle-matrix interactions. Steady shear rheological measurements on composites with different loading (0-30 vol %) were carried out with preshear treatment prior to the measurements, duration of kneading during compounding, and surface treatment (stearic acid) of the filler as variable parameters. The steady state shear viscosity massively increased with increasing filler volume fraction due to the presence of a small number of agglomerates. Although no polymer was entrapped within the agglomerates, their presence led to massive increase in shear viscosity. The theological response proved to be a more sensitive test for filler dispersion than scanning electron microscopy and shear thinning beyond that of the polymer matrix was observed, due to deagglomeration of the filler. The presence of agglomerates also led to a stress overshoot in the step shear rate experiments. Up to 0.3 filler volume fraction the particles agglomerated in form of clusters and no evidence for the presence of a space-filling particle network was found in this system. The apparent unbound viscosity observed at low shear stress in the high loaded composite and its fast decrease with increasing stress does not indicate yielding and can be attributed to gradual disintegration of the agglomerates. Preshearing the samples at a shear rate of 0.05 s(-1) prior to the measurements destroyed many of the agglomerates. Surface treatment of the particles decreased their surface energy and their tendency to agglomerate. It did not only decrease the particle-particle interactions but also decreased the adhesion of the polymer to the filler surface, resulting in a remarkable decrease in shear viscosity. The organic monolayer on the particulate surface lubricated the polymer flow, leading to an additional shear thinning at high shear rates. The maximum packing fraction phi(max) was estimated by fitting the relative viscosity eta(r) data to the Krieger-Dougherty equation. Reliable phi(max) values could only be obtained, when the data set contained eta(r) values larger than 3. (C) 2004 The Society of Rheology.