Miscible flow displacements in a rectilinear Hele-Shaw cell of Newtonian as well as rheologically well-characterized shear-thinning fluids are examined through experimental measurements and numerical modelling. Water is used as a displacing fluid while the displaced fluid consists of either a reference Newtonian glycerol solution or shear-thinning solutions of Alcoflood (TM) polymers of different molecular weights. The experimental measurements revealed that the shear-thinning behaviour of the non-Newtonian solutions resulted in more complex instability patterns and new finger structures not previously observed in the case of Newtonian displacements are identified and characterized. An analysis of the effects of the rheological behaviour of the shear-thinning fluids on instability characteristics such as the finger width and finger tip velocity is presented. Numerical simulations using a pseudo-spectral method are conducted and allowed to compare the predictions of the mathematical model based on an effective Darcy's law with the experimental measurements.