The magnetic interfacial needle stress rheometer is a device capable of sensitive rheological interfacial measurements. Yet even for this device, when measuring interfaces with low elastic and viscous moduli, the system response of the instrument contributes significantly to the measured response. To determine the operation limits of the magnetic rod rheometer, we analyze the relative errors that are introduced by linearly subtracting the instrument contribution from the measured response. An analysis of the fluid mechanics demonstrates the intimate coupling between the flow field at the two-dimensional interface and in the bulk at low Boussinesq number. A nonzero Reynolds number is observed to have a similar order of magnitude effect. The resulting nonlinear interfacial deformation profiles lead to an error, which depends on the magnitude of the interfacial modulus, as well as on the phase angle. The conditions under which reliable measurements can be obtained are identified. Based on the analysis of the effects of system response, the Boussinesq and Reynolds numbers, small modifications to the measurement probe are proposed. A reduction of the mass and localization of the magnetic material result in even further improved instrument sensitivity. This is demonstrated experimentally for two cases, i.e., a purely viscous interface of known viscosity, as produced by spreading thin silicon oil films on a water layer, and a time-dependent viscoelastic interface, generated by the surface gelation of a lysozyme solution. (C) 2008 The Society of Rheology.