Dielectric measurements were carried out for some methyl methacrylate (MMA) and styrene (St) related polymers and copolymers in dilute solutions at a frequency range from 1 kHz to 18 GHz. The dielectric data revealed the existence of three absorption regions. The absorption region associated with the side-chain relaxation for MMA-St copolymer with a low content of MMA units, showed a diffusion-controlled relaxation which can be interpreted in terms of Kramer’s theory but for large friction limits. On the other hand, all other polymers and copolymers investigated behaved according to Kramer’s theory but for small friction limits. This type of polymer was found to be little affected by solvent viscosity and its dipolar relaxation process is very similar to that caused by the internal rotation of the flexible chain. In addition to the relaxation associated with the side-chain rotation, two more distinct relaxation processes are detected. That in the low frequency region is found to be due to conformational rearrangement of the main chain, while that at higher frequency could be attributed to the inner mobility of the side groups.