Abstract: A polyisophthalate such as poly(ethylene isophthalate) and copolymers thereof having high intrinsic viscosities as well as reduced ring dimer of ethylene isophthalate, and a process for making the same. A protonic acid catalyst is utilized along with intrinsic viscosity modifying agents during preparation of the polyisophthalate. The polymers produced have very high clarity, improved oxygen and carbon dioxide barrier resistance, high glass transition temperature, as well as increased molecular weights. They are thus suitable for use as materials in packaging or forming containers, such as bottles.