By the application of high electric fields, small dilatations are induced in samples of polypropylene. These strains are detected at double frequency using a Michelson interferometer. This corresponds to the measurement of the proportionality factor between strain and the square of the electric field strength. Experimental results and a macroscopic discussion based on thermodynamics are presented. From X-ray experiments it is concluded that the majority of the investigated polymers can be regarded as isotropic, while two polymers show a slight anisotropy. With the exception of one material, the different isotactic polypropylenes show quite similar electrostrictive behavior. The purpose of the thermodynamic considerations is to study how the viscoelastic response of polymeric materials might influence the observed values.