Since commercial forced oscillation technique (FOT) devices became available, they have been widely used for physiological assessments, mainly of obstructive lung diseases. However, it is not known whether the impedance values measured with different devices are identical. In this study, two FOT devices-the impulse oscillometry system (IOS) and the MostGraph (MG)-were compared using phantom models. The resistance values varied up to 10% from estimated values in both devices. Additionally, there was a difference in frequency dependence for the resistance between the devices. The reactance values measured with MG were higher than those measured with IOS. The effects of ventilation on the measured impedance values were higher for IOS than for MG, especially at lower frequencies. We concluded that the devices do not always generate identical impedance values. Thus, differences between the devices should be taken into consideration when evaluating clinical data.