Our work shows, through measurements on polypropylene films, the validity of surface potential measurements as a technique for dielectric properties analysis. We propose a representation of the decay and potential return curves that separates different components of dielectric response, by using tdv/dt vs log t plots. Using this transformation, and varying some experimental conditions we have identified different contributions to the potential decay. From our experimental observations we suggest a qualitative interpretation of the initial seconds of potential decay for negative corona charged samples that uses a model controlled by surface trap emission and charge exchanges between corona discharge and the insulator surface.