An organic peroxide composition is provided which is liquid or near liquid at 25° C. or a low melting solid and which includes at least one ethylenically unsaturated organic peroxide (i.e., an organic peroxide containing at least one carbon-carbon double bond) and at least one saturated organic peroxide. The organic peroxide composition may further include at least one mono- and/or poly-unsaturated compound and at least one free-radical trap. The organic peroxide can be blended into a polymer such as a powdered or granular polyethylene resin. This peroxide-containing polymer can be used in rotational molding, wherein the polymer is added to a mold which is heated in an oven with rotation, thereby melting the polymer and coating the inside of the mold.