A method (100) for fixing at least one tissue sample (1) is carried out using a tissue processor (10). The tissue sample (1) is introduced at a first temperature level and a first pressure level into a fixing reagent (2) containing at least one nitrogen-containing compound capable of pressure- and/or temperature-dependent release of at least one aldehyde. The tissue sample (1) is left in the fixing reagent (2) for a fixing time period during which the fixing reagent (2) is brought to a second temperature level above the first temperature level and/or to a second pressure level above the first pressure level. A concentration in the fixing reagent (2) of the nitrogen-containing compound is ascertained using at least one concentration measuring device (14) of the tissue processor (10), and a signal is outputted on the basis of a measured value of the at least one concentration measuring device (14).