The invention relates to free-radical polymerization and free-radical cross-linking and more particularly provides a process of free-radical polymerization or cross-linking wherein at least one polymerizable monomer or at least one compound to be cross-linked is contacted in the presence of at least one selected organic peroxides, characterized in that the organic peroxide is prepared continuously by an ex situ process with the aid of a closed plate exchanger. In the process according to the invention, it's possible to introduce the above selected organic peroxide continuously during the polymerization or cross-linking reaction.