Models of ESD generators according to IEC 61000-4-2 are required for failure simulations in order to predict e.g. the robustness of ICs or the performance of overvoltage protection elements. Due to significant variations between different generator types a flexible and fast method for individual characterization is needed. A new method is proposed here based on impedance measurements at the discharge relay and the discharge tip. A current probe for high frequencies is used as transformer to overcome the problem of missing ground reference connection of the network analyzer at the discharge relay. The measured two-port impedance data in frequency domain can be used, after de-embedding of the measurement equipment influence, to approximate a state space representation in order to create a model for time domain network analysis. The proposed method can be extended with other ports to model e.g. field coupling into wires or PCBs. The Method is presented in this paper together with a detailed application example.