Abstract We have studied the magnetic properties of several R 2Co 17− x Ga x compounds with R=Y, Tb, Dy and Ho. X-ray diffraction measurements made on magnetically aligned powders of Y 2Co 17− x Ga x compounds showed that these materials retain their easy-plane magnetisation at room temperature up to at least x=5. The difference in behaviour with the corresponding Gd compounds and the absence of spin-reorientation phenomena in R 2Co 17− x Ga x compounds in which the R component has a negative second-order Stevens factor is discussed in terms of preferred site occupation of the Ga atoms. High-field measurements at 5 K were made on free powders of several Tb, Dy and Ho compounds. The intrasublattice constants derived from these data are somewhat smaller than those of the undiluted R 2Co 17 compounds, which is also discussed in terms of preferred Ga site occupation.