Electrodeposited NiPx alloys have been studied using electrochemical techniques, transmission electron microscopy and electron diffraction. The results show that crystals in the nanometer range are 'floating' in an amorphous phase. The crystal size is correlated with the catalytic activity of hydrogen evolution in an alkaline solution, it was found that the activity increased when the crystal size decreased. Electron diffraction patterns of the crystals showed that when few crystals were observed, they were found to be pure Ni. NI,P was found only when considerable amount of crystals had been formed. With the formation of Ni,P the activity was reduced. It is proposed that the increased catalytic activity is caused by an increased amount of amorphous phase surrounding the Ni crystals. This phase is able to absorb large amounts of hydrogen, thereby changing the electronic structure of the electrode material.