Structural and magnetic properties of the RFe11-xCoxTi compounds with R = Y and Er have been investigated. X-ray diffraction patterns and thermomagnetic curves show that all the synthesized compounds with x ranging from 0 to 11.0 are almost single phase and crystallize in the ThMn12-type structure. Substitution of Co for Fe leads to a monotonic decrease of lattice constants and unit-cell volume and a clear increase of the Curie temperature. The saturation moments increase with increasing Co content, going through a maximum at around x = 2.5, and then decrease with further increasing Co content. The easy magnetization direction (EMD) at room temperature for ErFe11-xCoxTi compounds is along the c-axis for x⩽4, perpendicular to the c-axis for 6⩽x⩽9 and then back to the c-axis for further increasing x. The anisotropy of the YFe11-xCoxTi compounds shows a similar behaviour. This concentration dependence of the magnetocrystalline anisotropy in the YFe11-xCoxTi and ErFe11-xCoxTi compounds results from the different contributions to the magnetocrystalline anisotropy from various transition-metal sites and the preferential occupation of Co atoms. A spin reorientation occurs below the Curie temperature for all ErFe11-xCoxTi compounds. Spin reorientation temperatures as a function of Co concentration were derived. A tentative spin phase diagram is given for ErFe11-xCoxTi compounds and can be understood in terms of crystal field theory.