A method is provided for monitoring the introduction of substitution fluids for an extra-corporeal blood processing device with an extra-corporeal blood circuit, comprising an arterial blood line, running from an arterial patient connection to a first chamber of a dialyzer or filter divided into said first chamber and a second chamber by a membrane and a venous blood line, running from the first chamber of the dialyzer or filter to a venous patient connection. A device is provided for monitoring the introduction of substitution fluids for an extra-corporeal blood processing unit with an extra-corporeal blood circuit and an extra-corporeal blood processing device with a device for monitoring the introduction of substitution fluids. The method and the device for monitoring the introduction of substitution fluids are based on monitoring the fluid level in the bubble trap, arranged in the venous blood line of the extra-corporeal blood circuit. The method and the device require that fluid is added to fill the extra-corporeal blood circuit, for example, for a rinsing procedure of the extra-corporeal blood circuit by means of the substitution fluid supply line arranged upstream or downstream of the dialyzer. A pre-dilution can be commanded when, after initiating the rinsing process the fill level in the bubble trap does not drop below a certain level. When by contrast the fill level drops below a given level a post-dilution can be commanded.