The viscoelastic properties of 30 vol.-% composites of calcite, surface-treated with fatty acids of different chain length, and polyethylene were studied. The alkyl chains decrease the surface energy of the particles and the adhesion between the polymer and the filler. The moduli of the composites decrease with decreasing interfacial adhesion. When the shear forces applied overcome the adhesion forces, interfacial slippage takes place. Terminal OH groups in the atkyl chains enhance the particles' agglomeration, leading to a solid-like response in the low frequency region. The presence of a plateau in the log moduli versus log frequency plot can be due to local structures and is not necessarily an evidence of a percolating filler network.