The effects of measuring procedures and activating additives on the electrorheological (ER) behaviour of hematite/silicone oil suspensions are analysed. The structures built up in the presence of an electric field without shear are stronger than those produced with both electric and shear fields simultaneously applied. Such differences are measurable when the field strength is not high enough to dominate over hydrodynamic interactions. Regarding the effect of additives, the ER response is enhanced by water until a certain maximum amount, beyond which the effect decreases. The increase in water concentration also leads to higher values of the electric current. Similar results are observed when Brij 30 is used. However, this surfactant only raises the yield stress at low fields. Contrary to water, the surfactant forms droplets in solution, instead of adsorbing on the hematite surface. At sufficiently high field strengths, the droplets can coalesce, enclosing the hematite particles and thus reducing the overall ER effect.