A combinatorial approach to deposit gas barrier layers (GBLs) on polyethylene terephthalate (PET) by means of plasma-enhanced chemical vapor deposition (PECVD) and plasma-enhanced atomic layer deposition (PEALD) is presented. Thin films of SiO_x and SiO_xC_yH_z obtained from PECVD were grown either subsequently on a PEALD seeding layer (SiO₂) or were capped by ultrathin PEALD films of Al₂O₃ or SiO₂. To study the impact of PEALD layers on the overall GBL performance, PECVD coatings with high macro defect densities and low barrier efficiency with regard to the oxygen transmission rate (OTR) were chosen. PEALD seeding layers demonstrated the ability to influence the subsequent PECVD growth in terms of the lower macro defect density (9 macro-defects mm⁻²) and improved barrier performance (OTR = 0.8 cm³ m⁻² day⁻¹), while the PEALD capping-route produced GBLs free of macro-defects.