Electroabsorption spectroscopy investigation and the determination of molecular parameters for rubrene dispersed in a poly(methyl methacrylate) (PMMA) matrix are reported. The features of the band system in the absorption spectrum in PMMA are analogous to those in solutions. The changes in the electric dipole moment and the polarizability between the excited and ground states are determined from analysis of the Stark effect in the absorption band. The change in the transition dipole moment in the presence of an external electric field is also observed. Although rubrene is predicted to be classified as a nonpolar molecule, there is a contribution of the difference in the electric dipole moment between the excited and ground states to the electroabsorption spectrum. The origin of the nonzero difference in the electric dipole moment is argued. Stark fluorescence spectroscopy investigation is reported in Part II of this series.