A method for deriving at least one operating parameter of a fluid-insulated electrical apparatus, in particular of gas-insulated switchgear. The operating parameter is dependent on a dielectric breakdown strength of an insulation fluid of the electrical apparatus. The insulation fluid includes at least three components that are assigned to at least a first and a second component group such that at least one component group comprises at least two components. The component groups differ in their weighted average values of the molecular masses of the components in the respective component groups. At least one quantity which is indicative of the concentration of the first component group and of the concentration of the second component group is determined from the insulation fluid, e.g. by measuring one or more measurement variables with one or more sensors. The operating parameter is then derived using the at least one quantity.