Multicomponent double layers, defined as layers composed of more than two kinds of charged particles, are supposed to constitute the predominant type of double layer in cosmic plasmas. A model of a steady and strong multicomponent double layer is studied in both the nonrelativistic and relativistic approximations. In particular, such properties of the layer as the structure, potential drop, and current composition are investigated. It is demonstrated that the density distribution of each kind of positive and negative particle in the nonrelativistic multicomponent layer is of the same shape as the density distribution of the ions and electrons, respectively, in Langmuir's two-component layer. Also, the shape of the distribution of the electric field corresponds to that of the two-component layer. In the relativistic layer the charges are distributed among two very thin layers of high positive and negative charge density close to the high-potential and low-potential boundaries of the layer, respectively, and a constant but low charge density in the rest of the layer. It is shown that the potential drop across the multicomponent layer is proportional to the thickness of the layer raised to 4/3 in the nonrelativistic case while it is directly proportional to the thickness in the relativistic case. Current conditions prescribing the allowed sets of current densities of the various kinds of particles in the layer are derived in the nonrelativistic and relativistic approximations. Correspondingly, abundance conditions for the particles accelerated through the layer are also obtained. The abundance of the particles accelerated by the double layer is expected generally to be different from the abundance of the ambient plasma. It is suggested that the abundance of the accelerated particles might serve as a means to detect double layers, especially in space plasmas. ¿ American Geophysical Union 1995