The influence of CO2 diffusion on eye formation during the ripening of an experimental Swiss-type cheese was studied. Three plastic films with defined CO2 permeability (198, 576, or 1050 cm3 m−2·24 h−1 bar−1) were compared to natural dry-ripening (control cheese without plastic film), testing two different cultures of Propionibacterium freudenreichii. Gross composition and volatile carboxylic acids were measured at the end of ripening. Physical parameters, such as eye volume, eye number, and eyeless border zones, were analysed by computed tomography. With increasing permeability, the relative eye volume decreased from 13.6% to 9.0% (p < 0.01). Surprisingly, the total number of openings increased from 570 to 711 (p < 0.05), which led to generally smaller openings with a mean value of ∼1000 mm3 compared to ∼2000 mm3 (p < 0.001). The control cheese with natural dry ripening showed a distinctive eyeless border zone with completely different diffusion properties compared to the samples ripened in films. The quantitative findings on eye formation dependent on plastic film permeability and dry ripening underscore the high importance of the diffusion barrier and are highly relevant for the cheese manufacturing industry.