The present invention relates to a method for determining the composition of medical liquids with regard to their fraction of electrolytes and non-electrolytes, comprising the steps of: determining at least one first physical parameter of the medical liquid for unambiguously determining the fraction of non-electrolytes in the medical liquid; simultaneously determining at least a second physical parameter of the medical liquid for determining the fraction of electrolytes in the medical liquid; and determining the fraction of electrolytes and non-electrolytes in the medical liquid based on the first and second physical parameter. The present invention further relates to an apparatus for determining the composition of medical liquids with regard to their fraction of electrolytes and non-electrolytes, comprising a first measuring apparatus (7) for measuring a first physical parameter of the medical liquid, which is unambiguously related to the non-electrolyte in medical liquid, a second measuring apparatus (8) for determining at least a second physical parameter of medical liquid, which is unambiguously related to the fraction of electrolytes in the medical liquid, and an evaluating apparatus (9) for determining the fraction of electrolytes and non-electrolytes in the medical liquid on the basis of the measured values of the first measuring apparatus (7) and the second measuring apparatus (8).