The invention relates to a method for determining the melt and/or slag level in a furnace vessel (1). In order to be able to determine in particular the slag height in the furnace vessel in a simple and accurate manner, the method according to the invention comprises the following steps: a) preparing a measuring probe (2) having a structure extending in a longitudinal direction, and in which probe (2) a cavity (3) is located that extends in a direction of the longitudinal axis (L) of the measuring probe (2); b) placing an optical waveguide (4) in the cavity (3) of the measuring probe (2), said optical waveguide (4) being placed in the measuring probe (2) so that it can be removed in a non-destructive manner; c) immersing the measuring probe (2) together with the optical waveguide (4) in the molten material (5) located in the furnace vessel, or introducing the measuring probe (2) together with the optical waveguide (4) into a part of the furnace wall of the furnace vessel (1), or introducing the measuring probe (2) together with the optical waveguide (4) into heat-isolating material located between the melt and the furnace wall of the furnace vessel (1), or introducing the measuring probe (2) together with the optical waveguide (4) into heat-dissipating material, all of which, or part of which, is located in heat-isolating material of the furnace vessel; and d) determining the temperature profile (T = T(x)) at least over a part of the length of the measuring probe (2) using fibre optic temperature measuring.