Grain boundaries are a source of weakness in Ni-based superalloys - thus for example they can be preferred fracture paths. Why is this? One reason particularly in the temperature range between 600 and 800 deg C is the role played by oxygen in causing a change in fracture mode form transgranular to intergranuar. Due to the application of these alloys in high temperature systems, for example the high pressure turbine discs used for jet propulsion and power generation, this issue is of technological significance. In this work, an electro-thermal-mechanical testing (ETMT) device is employed which is capable of the rapid assessment of the mechanical properties of such alloys; it makes use of miniature testpieces which are nonetheless of sufficient size to characterise macroscopicproperties. However, to interpret effects on the micro-scale, we have further characterised the ETMT testpieces by the preparation of micro-cantilevers which allows properties on the scale of the grain boundary to be assessed. This suite of experimental tools -- when combined with high resolution electron back scatter techniques to determine local geometrically necessary dislocation distributions - allow the microstructure-dependence of the mechanical properties to be elucidated.