The structure of the condensate film inside a closed two-phase thermosyphon is one of the key parameters concerning the counter-current flow limitation (CCFL). The CCFL is the most important performance limitation in technical applications of closed two-phase thermosyphons (with large length-to-diameter-ratios). Various physical models for the CCFL utilize coefficients associated with the condensate film structure (e.g. film thickness, wave amplitude). Sufficient data in this field are required to reliably predict the expectable thermosyphon performance. The present paper describes experimental work utilizing an annular closed two-phase thermosyphon equipped with a measurement system for the condensate film structure. The measurement principle is based on the capacitance change of a sensor installed flush with the wall which is covered by the condensate film. A very fast data acquisition system allows to detect fast variations in film thickness, i.e. surface waves. Experimental results with isopropylic alcohol as working fluid are presented and possible ways to implement the results into existing flooding models are discussed.