The anesthetic agent propofol is applied intravenously and different groups demonstrated that it is detectable in breathing gas. To quantify the propofol concentration in breathing gas (c breath ) might be a promising feedback for anesthesiologist and for potential closed loop control, yet there is no online measurement in standard clinical practice. Since the physiological relevance of the propofol concentration in breath is not entirely known it may be adverse to control the infusion with c breath as target variable. In order to control the concentration at the plasma site (c plasma ) or even at the effect site (c effect ) in the brain mathematical models can be used to describe the dependencies between c breath and c plasma or c effect . This contribution presents the pharmacokinetic modeling of the transition from blood to alveolar gas concentration of propofol. For characterization a model described by a gas blood partition coefficient and one time constant or an equivalent one compartment system, respectively, is taken into account. Clinical data obtained in a study with 17 patients are used for fitting. During anesthesia breathing gas was monitored continuously with an electrochemical sensor and venous blood samples were taken at given times. The use of the mentioned model structure leads to a simple and adequate characterization. A data conditioning in form of a model based interpolation was performed prior to the identification process. The identified parameters are comparable to results of other research works.