Device for monitoring the supply of substitution fluid for an apparatus for extracorporeal blood treatment, wherein the extracorporeal blood treatment apparatus comprises: an extracorporeal blood circuit, which comprises a first chamber of a dialyser or filter divided by a membrane into the first chamber and a second chamber, and a fluid system, which comprises the second chamber of the dialyser or filter, a device for supplying blood to the first chamber of the dialyser or filter at a specific blood flow rate Q B, a substitution device for supplying substitution fluid to the blood upstream or downstream of the dialyser or filter at a specific substitution rate Q S and an ultrafiltration device (13, 14) for withdrawing fluid from the blood via the membrane of the dialyser or filter at a specific flow rate Q M, characterised in that the device for monitoring the supply of substitution fluid comprises: a measuring unit for measuring the density of the blood or a blood constituent in the blood circuit of the dialyser or filter, a control unit for controlling the substitution device and/or the device for supplying blood and/or the ultrafiltration device (13, 14), wherein the control unit is designed in such a way that substitution rate Q S and/or blood flow rate Q B and/or flow rate Q M of the fluid withdrawn through the membrane of the dialyser or filter is changed, and an evaluation unit which comprises a comparison device that generates the difference between the value for the density of the blood or a blood constituent measured before the change in the delivery rate and the value for the density of the blood or a blood constituent measured within a specific time interval after the change in the flow rate and compares said difference with a preset threshold value in order to be able to ascertain an increase or decrease in the density of the blood or a blood constituent by a preset amount, wherein the evaluation unit is designed such that it is concluded, on the basis of the amount and/or the direction of the change in density of the blood or a blood constituent, whether a dilution is taking place and whether a pre-dilution or post-dilution is present. an extracorporeal blood circuit, which comprises a first chamber of a dialyser or filter divided by a membrane into the first chamber and a second chamber, and a fluid system, which comprises the second chamber of the dialyser or filter, a device for supplying blood to the first chamber of the dialyser or filter at a specific blood flow rate Q B, a substitution device for supplying substitution fluid to the blood upstream or downstream of the dialyser or filter at a specific substitution rate Q S and an ultrafiltration device (13, 14) for withdrawing fluid from the blood via the membrane of the dialyser or filter at a specific flow rate Q M, the device for monitoring the supply of substitution fluid comprises: a measuring unit for measuring the density of the blood or a blood constituent in the blood circuit of the dialyser or filter, a control unit for controlling the substitution device and/or the device for supplying blood and/or the ultrafiltration device (13, 14), wherein the control unit is designed in such a way that substitution rate Q S and/or blood flow rate Q B and/or flow rate Q M of the fluid withdrawn through the membrane of the dialyser or filter is changed, and an evaluation unit which comprises a comparison device that generates the difference between the value for the density of the blood or a blood constituent measured before the change in the delivery rate and the value for the density of the blood or a blood constituent measured within a specific time interval after the change in the flow rate and compares said difference with a preset threshold value in order to be able to ascertain an increase or decrease in the density of the blood or a blood constituent by a preset amount, wherein the evaluation unit is designed such that it is concluded, on the basis of the amount and/or the direction of the change in density of the blood or a blood constituent, whether a dilution is taking place and whether a pre-dilution or post-dilution is present. a measuring unit for measuring the density of the blood or a blood constituent in the blood circuit of the dialyser or filter, a control unit for controlling the substitution device and/or the device for supplying blood and/or the ultrafiltration device (13, 14), wherein the control unit is designed in such a way that substitution rate Q S and/or blood flow rate Q B and/or flow rate Q M of the fluid withdrawn through the membrane of the dialyser or filter is changed, and an evaluation unit which comprises a comparison device that generates the difference between the value for the density of the blood or a blood constituent measured before the change in the delivery rate and the value for the density of the blood or a blood constituent measured within a specific time interval after the change in the flow rate and compares said difference with a preset threshold value in order to be able to ascertain an increase or decrease in the density of the blood or a blood constituent by a preset amount, wherein the evaluation unit is designed such that it is concluded, on the basis of the amount and/or the direction of the change in density of the blood or a blood constituent, whether a dilution is taking place and whether a pre-dilution or post-dilution is present.