Abstract. During the S4 LSC science run, the gravitational-wave detector GEO600, the first large scale dual recycled interferometer, took 30 days of continuous data. An instrumental duty cycle greater than 96% and a peak sensitivity of 7 × 10⁻²²/√Hz around 1 kHz were achieved during this time. Detector commissioning and characterization work are essential to prepare the worldwide network of gravitational-wave detectors for future extended science runs. This paper describes the detector commissioning that was done in the run-up to S4. The focus is set on techniques used for the identification and removal of limiting noise sources. Furthermore we give some examples for the detector characterization work of GEO600.