First and second dielectric constants, e.sub.1 and e.sub.2 respectively, for first and second dielectric materials forming a MOS (metal oxide semiconductor) stack are determined. First and second test MOS stacks having first and second total effective oxide thickness, EOT.sub.A and EOT.sub.B, respectively, are formed. The first and second test MOS stacks include first and second interfacial structures comprised of the second dielectric material with first and second thickness, T.sub.2A and T.sub.1A, respectively. In addition, the first and second test MOS stacks include first and second high-K structures comprised of the first dielectric material with first and second thickness, T.sub.2B and T.sub.1B, respectively. The thickness parameters EOT.sub.A, T.sub.1A, T.sub.2A, EOT.sub.B, T.sub.1B, and T.sub.2B of the test MOS stacks are measured. The dielectric constants, e.sub.1 and e.sub.2, are then determined depending on relations between values of EOT.sub.A, T.sub.1A, and T.sub.2A, and between values of EOT.sub.B, T.sub.1B, and T.sub.2B.