In a combined motor transducer (CMT) design, the torque is measured at the moving surface and the moment of inertia of the moving parts of the instrument and the fluid inertia might influence the measurement results, whereas in a separate motor transducer (SMT) setup only fluid inertia effects might come into play. In order to investigate inertia effects, oscillatory shear measurements have been performed on various Newtonian liquids with low viscosity with different measuring geometries and in a CMT and a SMT setup. For a CMT situation, it is shown that with an appropriate control mechanism, taking into account the instrument inertia reliable results can be achieved up to rather high ratio of acceleration to sample torque. Further, the measuring data on the Newtonian liquid agree well with the fluid inertia calculations following the approach from Schrag [Trans. Soc. Rheol. 21, 399-413 (1977)]. The results for a SMT setup show that for any given geometry, fluid inertia effects, when uncorrected, lead to a frequency dependent baseline in the elastic modulus G'. The larger the measuring gap the larger the G'. Combining the findings for both the CMT and the SMT situation, it can be concluded that for Newtonian liquids and fluids with low elasticity fluid inertia adds an elastic torque which in CMT points in opposite direction of the elastic part of the sample torque, whereas in SMT the additional elastic torque, while its absolute value is somewhat smaller, acts in the direction of the elastic part of the sample torque, respectively. To investigate weakly structured fluids, it is suggested to use geometries with rather small measuring gaps, and to determine the individual baseline for the geometry with Newtonian liquids. (C) 2016 The Society of Rheology.