The structure and magnetic properties of LiNi 0.5 Mn 0.5 O 2 were studied by synchrotron X-ray diffraction, dc and ac susceptibilities. The material showed a continuous magnetic transition from paramagnetism into ferrimagnetism, followed by a spin glass with the decrease of temperature. Using a criterion given by Banerjee to distinguish first-order magnetic transition from second-order ones, it is shown that the ferrimagnetic transition at 108K belongs to the second-order type. The frequency dependence of peak intensity and the shift in ac susceptibility at 14K suggest a reentrant spin glass transition in LiNi 0.5 Mn 0.5 O 2 .