Intensity (ILS) and dynamic light scattering (DLS) experiments were performed on semidilute acid aqueous solutions of unmodified chitosan (UM-chitosan) and of hydrophobically modified chitosan (HM-chitosan) with two different degrees of C12-aldehyde substitution. Both the ILS and the DLS measurements suggest the formation of association structures. According to the ILS measurements in the range q xi > 1 (q is the wave vector and xi is the correlation length), all the systems have a fractal structure and the fractal dimension is about 2. The time correlation data obtained from the DLS experiments revealed, for all systems, the existence of two relaxation modes, one single exponential at short times followed by a stretched exponential at longer times. The value of the slow relaxation time increases with increasing concentration and hydrophobicity. This behavior reflects the importance of intermolecular associations and hydrophobic interactions. The reduced slow relaxation time exhibits a weak temperature dependence for both UM-chitosan and HM-chitosans. The fast mode is always diffusive, while the slow mode shows an approximately q(3) dependence at low concentrations and a q(2) dependence, characteristic of diffusion, at higher concentrations.