The standard industrial process to produce medium-voltage electric cables based on EPDM consists of cross-linking by peroxides with high-temperature steam (pressurized water vapor). Suboptimal material cross-linking is usually due to a decrease of the temperature along the vulcanization pipe. Temperature variations are connected to variations in steam pressure into pipe system. A combined numerical and experimental approach to optimize the production process of medium-voltage, rubber-insulated electric cables vulcanized with steam water is presented. The numerical part of this process is based on the use of finite elements and an optimization genetic algorithm (GA) and will be presented in Part 2. In Part 1, attention focuses on the experimental investigation. In particular, the final cross-linking degree is experimentally obtained by means of differential scanning calorimetry (DSC) determination of nondecomposed peroxide from the external layer to the core of the cable insulation. The final task is to minimize the difference between numerically predicted and experimentally determined cross-linking degree using a steam-temperature profile along the pipe to explain the variations. A preliminary evaluation of kinetic-reaction constants of rubber cured with peroxides is provided with the support of a comprehensive experimental investigation of the curing process by means of standard rheometer characterizations done at different curing temperatures. An existing mathematical, kinetic model is applied to the experimentally determined rheometer curves, allowing the determination of partial-reaction kinetic constants used in the finite-element computations.