The structure and the magnetic properties of ErCo10-xNixSi2 alloys with (0 less than or equal to x less than or equal to 10) have been investigated by means of X-ray powder diffraction and dc magnetization measurements. All the compounds in the series crystallize in the tetragonal ThMn12-type structure with the space group symmetry I4/mmm. Substitution of Ni for Co does not change the crystal structure but leads to a decrease in the unit-cell volume. With increasing Ni content, the magnitude of the Co moments decreases strongly. A similar pronounced decrease is displayed by the Curie temperatures. The compensation temperatures were observed for the compounds with x = 4 and 6. In the ErNi10Si2 compound, the magnetization of the Er sublattice decreases strongly since there is no molecular field produced by the 3d sublattice magnetization in this compound. This indicates that the magnetization of the Er sublattice is strongly affected by the molecular field produced by the magnetization of the 3d sublattice. (C) 2004 Elsevier Ltd. All rights reserved.