Nanoparticles of pure Ni and Ni–Cu solid solution were synthesized by the sol–gel method at 400–600 °C. The particle sizes and values of specific surface of powders were dependent on the synthesis temperature. The magnetization of the obtained powders was studied using a high-temperature magnetic balance. The Curie temperature for particles of pure nickel did not depend on their size but the specific magnetization at 300 K decreased linearly with increasing specific surface area of particles. For particles of the Ni–Cu binary solid solutions, the Curie temperature and values of magnetization increased with decreasing particle size for particles with diameters up to values less than 100 nm. The amount of adsorbed copper on the particles surface, the change of nickel concentration in the volume of the particles and the Curie temperatures, depending on the Cu (Ni) concentration in a volume of particles, were calculated using the Gibbs equation. The calculations made in the framework of a perfect solution showed qualitative agreement between experimental and calculated values of the Curie temperature for the system Ni–Cu.