Abstract: Chemically modified olefin polymers are made by reacting solid particles of ethylenically unsaturated olefin polymer with an oxy-addition agent in a compatible non-solvent liquid. The liquid is one which wets the polymer but does not dissolve or soften it. The polymer is particularly polypropylene and especially substantially isotactic and thermoplastic unsaturated polypropylene. The oxy-addition agent is particularly an epoxidizing agent such as hydrogen peroxide or a percarboxylic acid. Useful modified polymers include epoxidized polyolefins which are more polar than the parent polyolefins and can be used as intermediates in making other derivatives such as diols, ethers and esters.