In this work the influence of thermal treatments on the structural and electronic properties of synthetic melanins was investigated. To that end, thermal gravimetry, infrared spectroscopy, electron spin resonance, and dc conductivity were employed. Our results indicate that the water corresponds to 10%–20% of the mass of synthetic melanin and also show that the latter is rich in mobile interlayer hydrogens or protons. Extended heating above 60°C decreases the conductivity by three orders of magnitude and increases the spin density by a factor of 3. This enhanced spin concentration and decreased conductivity are stable at room temperature, but can be reestablished by rehydration. Evidence is given that these properties are related to the presence of mobile interlayer protons. Prolonged heating is responsible for proton trapping by carboxylic groups forming COOH moieties, leading to the changes observed in electronic properties.