We present a new approach to determine quantitatively the phase content of multiphase polymers by applying a spin-diffusion pulse sequence (filter) prior to cross-polarization (CP) in a C-13-CPAS experiment. The technique is exemplified for two rather different polymer systems, a high-density polyethylene (HDPE) and four styrene-isoprene diblock copolymers, all of known phase contents. In principle, the technique should be applicable to any multiphase system in which spin-diffusion between different regions/phases exists and where the magnetization of phases can selectively be filtered out.