We report on the viscosity in the supercooled liquid region of mechanically alloyed Zr55Cu30Al10Ni5 glassy matrix composites containing increasing volume fractions of CaO or ZrC as a second phase. Results obtained by parallel plate rheometry are compared with single-phase amorphous Zr55Cu30Al10Ni5 containing small carbon or calcium additions (up to 4 at.%). This allows an estimation of the effect of different matrix compositions, resulting from limited particle dissolution processes occurring upon milling, on the viscous flow behaviour of the supercooled liquid and on the amount of free volume of the glassy phase. Once dissolution processes are taken into account, the contribution of second phase particles on the overall viscosity is discussed considering the Einstein equation for the flow of mixtures.