The electrical characteristics of electrooptic polymer waveguide modulators are often described by the bulk reactance of the individual layers. However, the resistance and capacitance between the layers can significantly alter the electrical performance of a waveguide modulator. These interface characteristics are related to the boundary charge density and are strongly affected by the adhesion of the layers in the waveguide stack. An electrical reactance model has been derived to investigate this phenomenon at low frequencies. The model shows the waveguide stack frequency response has no limiting effects below the microwave range and that a true dc response requires a stable voltage for over 1000 h. Thus, reactance of the layers is the key characteristic of optimizing the voltage across the core layer, even at very low frequencies (>10-6 Hz). The results of the model are compared with experimental data for two polymer systems and show quite good correlation.