A copolymer based on alpha -methylstyrene (AMS) was investigated by nuclear magnetic resonance (NMR). The styrene-co-alpha -methylstyrene (SAMS) was analyzed by solution and solid-state NMR techniques. Three copolymers of SAMS with different compositions presented a particular behavior. The solution results showed the copolymer microstructure and the AMS content. The carbon-13 spectra of SAMS C indicated that the AMS CH3 signal was detected at three distinct chemical shifts, because of the different comonomer-sequences distribution. The proton spin-lattice relaxation time in the rotating frame (T(1)(H)rho) parameter was chosen because it permits the evaluation of changes in the molecular mobility. The values of T(1)(H)rho found for the copolymers confirmed the random distribution in the samples. The copolymer with a low quantity of AMS (1.7%), when analyzed by this relaxation parameter, showed lower values that were interpreted as an antiplasticization effect. The SAMS copolymer with a higher AMS quantity showed a plasticization effect.