Helical coil steam generators feature excellent heat transfer performance, compactness and mature manufacturing technology. In most designs, the two-phase mixture flows on the shell side of the heat exchanger, however in the steam generator design adopted in advance reactors such as the NUSCALE reactor design, the two-phase flow mixture flows in the helical coils of the heat exchanger. To improve the computational models used to predict the performance of such steam generators, it is necessary to characterize the behavior of two-phase flows in helical coils. In the present paper, detailed measurements of the void fraction distribution of an air-water two-phase flow in a glass transparent helical coil were conducted. At this aim, an X-ray radiography and high-speed videography system were used. The high-resolution measurements were also used to identify and classify flow regimes. A total of 15 runs is presented in this paper, classified into 6 flow regimes, namely: bubbly flow, plug flow, stratified wavy flow, slug flow, semi annular flow and annular flow.