The solubility of a typical UV-absorbing stabiliser has been measured in a series of poly(ester-block-ether)s copolymers (PEBE) with different ratios of hard and soft blocks. The solubility is higher in the PEBEs than in polypropylene and increases with the amount of polyether in the PEBE. The composition dependence suggests that the additive is at least ten times more soluble in the soft polyether phase than in the semi-crystalline hard phase. A thermodynamic analysis in terms of regular solution theory shows that increased solubility is due to better compatibility of the additive with the polyether than with PP, mainly due to much more favourable heat of mixing.