X-Ray line broadening and adsorption studies on various types of Raney nickel prepared or treated at temperatures of 50, 80, and 107 °C have shown an approximately linear relationship between surface area and the reciprocal of the crystallite size. The low-temperature preparations exhibited a greater concentration of residual crystalline alumina trihydrate. Removal of the crystalline alumina at 50 °C, in some cases, led to increased porosity and surface area without changing crystallite size. More severe preparation or treatment in aqueous alkali gave catalysts of lower total aluminum and alumina trihydrate content. These conditions caused an increase in crystallite size, which was greater than that in catalysts from comparable heat treatment in vacuo, and emphasizes the importance of chemical as well as thermal processes in determining the nature of Raney nickel.