Abstract The extraction constants (Kexn = [LnX3·nE]o/[Ln3+][X−]3[E]on, n = 3 and/or 4) were determined across the lanthanoid series, when lanthanoids(III) were extracted from a 1 M sodium thiocyanate (NaX) solution into benzene and chlorobenzene containing trioctylphosphine oxide (TOPO, E). The dominant species were LnX3·4E for light lanthanoids and LnX3·3E for heavy ones. Although both extraction constants increased along with the atomic number, the difference in the magnitude of log Kex4 from log Kex3 to a less-polar solvent, benzene, was larger than that to chlorobenzene. The separation factor was similar either in light lanthanoids or in heavy lanthanoids to both solvents, reflecting the similar variation of the constants across the series. However, in the middle of the series, it was smaller upon using less-polar solvents than polar solvents. One of the reasons was the larger proportion of LnX3·4E, of which extraction constant did not change much in the middle of the series. In addition, when the concentration of TOPO was not in a large excess of the lanthanoid(III) concentration it was due to a greater consumption of TOPO; that is, the heavier lanthanoid(III), which was extracted better, required more TOPO molecules to be combined with LnX3, and into less-polar solvents lanthanoids(III) were extracted better than into polar solvents. Consequently the separation factor when using less-polar solvents was smaller than expected from the extraction constants.