A bi-directional flow impulse type turbine arrangement (120) for use with a bi-directional reversing flow (F, F1, F2), and in particular for use with an oscillating water column power plant (110). The turbine arrangement (120) has a rotor (120), and first and second sets of guide vanes (140, 142) located on an opposite axial sides of the rotor (132) for directing the bi-directional reversing flow (F, F1, F2) to and from the rotor (132). The guide vanes (140, 142) are disposed at a greater radius and radially offset from the rotor blades (136). The turbine arrangement further comprises first and second annular ducts disposed respectively between the first and second sets of guide vanes (140, 142) and the rotor (132) for directing fluid from the guide vanes (140, 142) to the rotor blades (136). In addition, and in other aspects of the invention, the vanes (140,142) and rotor blades (136) have a novel profile, with in particular the vanes (144) having a profile to maintain a constant flow passage between the vanes over a turning section of the vanes, and the rotor blades of the rotor (132) having a higher turning angle (2alpha), and producing a non-axial outlet downstream flow. The turbine (120) may also in another aspect incorporate vanes with a slot and a boundary layer blowing arrangement to re-energise and improve the reverse flow over the vanes.