Electrical resistivity, AC magnetic susceptibility, X-ray powder diffraction, EDAX and EELS near edge absorption, have been used to study a series of Ni substituted dense (mass density approximately 6.0 g/cm³) superconducting YBa₂Cu_3-xNi_xO_{7- delta} with a wide range of x values. T_c was found to decrease linearly with increasing Ni concentration. The authors have also found phase separation occurring at high Ni concentration due to the formation of Y₂BaCuO₅, Y₂BaNiO₅ and NiO/BaO which seriously modifies the Ni concentration from its nominal values. For this reason many earlier measurements on the effects of Ni substitution on the physical properties of YBa₂Cu₃O_{7- delta} may need to be re-examined and this paper attempts to correlate the above physical properties with actual Ni concentrations in the samples.