The development of long-term implantable microsystems requires biocompatible packaging. Although several approaches using biocompatible polymers for encapsulation of flexible electronics have been demonstrated, there has not been found a hermetic solution yet as it is a state of the art for rigid titanium or ceramic packages. Besides numerous, advantageous properties for flexible microsystem packaging, polymers are just partly able to provide a sealed encapsulation for long-term implantation due to their permeability to water and certain ions. This paper proposes a novel in vitro test setup to characterize the behavior of polymeric encapsulation and hence their protective function for electronics in artificial body fluids. That allows monitoring the variation of values of the impedance and leakage current of specimen, which were placed under fluidic and electrical stress for a test period of 30 days. The protective function of polyimide PI 2611 and epoxy resin EPO-TEK 310M has been investigated, with the polyimide showing the best performance.