Dielectric and IR measurements were carried out for copolymers of isopropyl methacrylate with methacrylic or acrylic acid at various content units. The acid units introduced into isopropyl methacrylate polymer chain formed acid dimers and free acid units in the neighborhood of isopropyl methacrylate units. According to the segmental mode relaxation of main chains (α process), the motion of main chains was found to be restricted by acid dimers formed among acid groups, and reinforced by the free acid units. The acid dimers and free acid units acted even on the local mode relaxation of isopropyl methacrylate side chains. The relaxation strength of the α process (Δα) for copolymer samples increased with temperature. The increase in Δαwas attributed to an increase in the number of motional units caused by breakdown of the acid dimmers. The dissociation energy (ΔH0) of the acid dimmer could then be estimated from the temperature dependence of Δεα.