The relationship between the structure and the viscoelastic properties of resol resins was studied. Six phenolic resins (resol) were synthesized with different molar ratios of formaldehyde to phenol. These resols were cured by means of temperature and without catalyst. The characterization of the resol was done by means of infrared spectroscopy and chemical methods. From the viscoelastic properties of fully cured resins, characteristic properties such as: storage modulus (E＇), tan delta, width of the tan delta(Delta T) and damping peak (tan delta) were obtained. The compression modulus and the void content were also determined. A maximum in the methylene bridge and the result of the viscoelastic properties allow us to say that the resol with F/Ph between 1.3 and 1.4 has the highest crosslinking density.