A method for the detection of contaminants in an optical measuring cuvette of a spectrophotometer, typically an oximeter for determining hemoglobin derivatives, is provided, in which measuring cuvette, in addition to at least one sample measurement to obtain a sample spectrum I(λ), at least one reference measurement is performed using a reference liquid to obtain a reference spectrum I0(λ). The reference spectrum I0(λ) is compared to a known target spectrum I0soll(λ), which is associated with the measuring cuvette, comparison parameters being obtained, it being decided automatically whether a contamination of the measuring cuvette exists as a function of predefined threshold values of the comparison parameters.