The formation, structure and transport properties of LixNi1-xO solid solutions with x less than or equal to 0.3 are described. Pure stoichiometric nickel oxide is a green insulator compound with rocksalt structure. When Li+ partially substitute the normal sites of Ni2+, a hole is introduced in the form of Ni3+ or O- to keep charge neutrality conditions, and the oxide becomes a black semiconductor. For Li atomic fraction x > 0.3 a rhombohedral distortion of the cubic structure by partial lithium and nickel ions ordering on alternate (111) planes takes place, giving rise to a hexagonal structure. Some questions, regarding the hole state, the ordering and vacancy presence in the solid solutions, are discussed. The effect of Li content in the solid solutions on the most typical charge-and-mass transport properties, as electrical conductivity, thermoelectric power and chemical diffusivity, is reviewed. (C) 2003 Elsevier B.V. All rights reserved.