Copolymerization and terpolymerization of three methacrylic monomers, 3-hydroxyneopentyl methacrylate (NPGMA), 2-hydroxyethyl methacrylate (HEMA) and methyl methacrylate (MMA), have been studied. Attention has been devoted to quantitative analysis of the experimental results for determining the reactivity ratios, which were obtained by the Kelen-Tudos and Tidwell-Mortimer methods in the copolymerization reactions. A method developed in our laboratores to determine the reactivity ratios (r(ij)) in multicomponent systems was used in the terpolymerization of these monomers. The values of r(ij) obtained were different from those previously calculated by binary copolymerizations. Azeotropic reactions occur in the case of MMA-NPGMA copolymerization and in the terpolymerization extending over practically all the interval of composition. Microstructural effects on the glass transition temperature (T-g) of HEMA-NPGMA copolymers and terpolymers were observed, indicating the influence of sequence distribution of the monomer units on that property. In the other copolymers the values of T-g were not influenced by the sequence distribution and are given directly as a function of the weight fraction of the comonomer.