Investigations of nickel hydride formation were carried out using in situ X-ray. diffraction (XRD) during cathodic charging of Ni in 2 M H2SO4 solution with and without thiourea. In the process of cathodic charging the XRD peak (200) of the phase beta-NiH0.7 was determined to shift towards lower Bragg diffraction (2 Theta) angles and go back within 3-5 min following interruption of charging. The shift corresponded to changes in the lattice parameter a of the beta-NiHx phase. The a value depended on the cathode potential E-c or the charging current density i(H) and the quantity of thiourea in the solution, a always fell back to 0.3725-0.3728 nm after interruption of charging. Depending on E-C or i(H), the change in x during cathodic charging was observed within the following ranges : 0.7 less than or equal to x less than or equal to 0.73 without thiourea, 0.7 less than or equal to x less than or equal to 0.76 with thiourea concentration being 0.2 g dm(-3) and 0.7 less than or equal to x less than or equal to 0.81 with 5 g dm(-3) thiourea in the solution. It is concluded that the x value of beta-NIHx is related to the variations of the lattice parameter a.