Experimental results of the triboelectric charging of insulators have variously been interpreted in terms of either electron or ion exchange, but the field is fraught with contradictions and it remains one of the least understood areas of solid state physics. We approached this contentious issue from the point of view of charge penetration depth (CPD) into the surfaces of insulators. Hitherto, it appears that the questions of charging mechanisms of insulators and charge penetration depth into insulators have been treated separately. We show how these are related in the form a coherent model for metal-insulator and insulator-insulator charging. Using a polymer chemistry approach, combined with XPS surface analysis, we designed a series of polymer films such that their surface compositions were different from those of the bulk. We found that charging against these films fell into two distinct patterns depending on whether the charging was with metals or other insulators. Charging with insulators related to the topmost surface layers of the films, but charging with metals related to the deeper levels of the bulk compositions, clearly indicating that different mechanisms MUST be involved. We propose that charging against insulators involves ion adsorption to the topmost atomic layers of the films; and charging against metals involves electron tunneling deeper into the bulk of the films. Others have proposed electron and ion exchange mechanisms using different systems for each but, as far as we know, this work is the first to invoke both electron AND ion exchange mechanisms for the same insulating films; and it is for this reason we believe that the approach described here, together with advanced surface analysis techniques, provides the scope and flexibility for precise quantification of charge penetration depths, which could lead to totally definitive evidence for electron versus ion involvement. Integrating our work with that of others has resulted in modification of - - the above model such that both electron and ion exchange mechanisms exist simultaneously for metal-insulator charging. A specific experimental approach is identified to verify this updated model. Part of this work was conducted at Xerox Corporation and presented at the 1975 IEEE meeting in Atlanta, GA (1). Here it is expanded and interpreted in the light of recent developments.