[en] The U-tube region in steam generators is prone to excessive vibration due to two-phase cross flow. Two-phase flow-induced vibrations depend not only on the velocity of the flow but also strongly on void fraction and flow pattern. Thus, it is necessary to understand the dynamic characteristics of two-phase flow and to be able to predict flow patterns. Currently a limited amount of data on flow patterns and void fraction exists due to the complexity of the flow and the difficulties in taking useful measurements. A research program was undertaken to help this understanding. The program involves the development of fiber optic probes and capacitance sensors to measure the detailed characteristics of the flow such as local void fraction, gas flow velocity, void size and void passage frequency. The measurement of these characteristics provides the necessary information to define and identify objectively two-phase flow patterns. The measurement of void fraction is a useful technique to identify flow patterns. This paper describes the development of capacitance sensors to measure two-phase cross flow void fraction in a tube bundle. (author)