We report on a Terahertz magnetospectroscopy study of a set of five HgTe quantum wells of different thickness, from below to above the critical thickness d c . In quantizing magnetic fields up to 11 T, both intraband and interband transitions have been observed. In samples with inverted band structures, we confirm the observation of the crossing avoiding of the zero-mode Landau levels at a critical value of the magnetic field. In samples with non-inverted band structures, close to the critical thickness, we report on the square root dependence of the intraband transition energy on the magnetic field, as expected in the single-particle model of massless Dirac fermions. The obtained results are compared with the allowed transition energies calculated using the 8 × 8 Kane model.