We present results from a study of buoyant miscible displacements flows at moderate viscosity ratios in near-horizontal ducts. Low Atwood numbers are considered. Inertial effects are significant although the flow regimes are laminar. Pipe and plane channel geometries are considered using a mix of experimental, computational and analytical techniques. The main focus of the study is on the effects of the viscosity ratio between the fluids. We show that small viscosity ratios lead to more efficient displacements, as is intuitive. In each geometry we find a mix of viscous and inertial flows, in broadly the same pattern as for the iso-viscous displacements studied extensively in Taghavi et al. (submitted for publication). Predictive models are proposed for the viscous regime, in the case of the plane channel, and for the inertial exchange flow regime, in both geometries. We also study displacement flows with shear-thinning fluids, over a more restrictive range of parameters. We show that with an appropriate definition of the effective viscosity the scaled front velocities fit well with the results from the Newtonian displacements, in both pipe and plane channel geometries. The main role of ratio m of displaced fluid viscosity to displacing viscosity is in line with our intuition. For m > 1 displacement efficiencies are reduced and the front velocities are larger. However, the main increase in front velocity is achieved for modest viscosity ratios of 3 or 4 to 1, with little increase afterwards. The reverse situation, m < 1, shows significant improvements in displacement efficiency, with front velocity <(V)over cap>(f) reduced down towards the imposed mean velocity (V) over cap (0), as m decreases. A viscosity ratio of around 1-4 appears to achieve quite efficient displacements, and to be able to compensate for the effects of buoyancy (which in our case are always destabilising). (C) 2011 Elsevier Ltd. All rights reserved.