In this paper, the mechanisms that lead to the conversion of common-mode (CM) RF interference (RFI) into differential-mode (DM) disturbances, which corrupt the information content of nominal signals and impair the operation of electronic systems, are investigated. To this purpose, distributed and lumped common-mode into differential-mode (CM-DM) conversion mechanisms are discussed with reference to a simple test structure that can be analytically described. On the basis of the proposed analysis, the origin and the relative impact of such mechanisms is highlighted and the detrimental effect of distributed CM-DM conversion on the effectiveness of differential RFI suppressing filters is discussed. Theoretical predictions are compared with experimental results.