In bubble column reactors mass transfer kinetics are determined by local hydrodynamic processes (bubble formation, velocity, coalescence, break-up, etc.). For a better understanding of the influence of pressure and particle load on these processes, a measurement system for detection of local bubble size distributions in opaque bubbly flows is needed. Therefore, in this work an optical needle probe was applied in a bubbly flow. In a first step, the needle probe was validated by means of single-bubble measurements. The measurement technique shows high potential for application in more complex and opaque systems.