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Orthotics are part of the immense medical devices industry; their aim is to improve the patient’s life by correcting or treating injuries. Therefore, it is of great importance to understand the physical, chemical and mechanical properties of the materials used to develop these products. This research uses techniques such as X-ray diffraction, thermogravimetric and differential scanning calorimetry analyses, Fourier transform infrared spectroscopy and scanning electron microscopy, for the characterization of ethylene-vinyl acetate (EVA) used for manufacturing orthopedic insoles in Costa Rica. These techniques allowed to identify specific characteristics of two EVA samples with different value of hardness in the Shore scale, such as the presence of calcium carbonate (CaCO3), the amount of vinyl acetate in the compound, a melting temperature of 72° C, and differences between the material’s densities. The results show that the main reason in the variation of hardness between both samples is the amount of vinyl acetate present which can be increased or reduced according to specific requirements.