The concept of translaminar fracture toughness of 0° plies has enabled the development of a considerable number of ply-level numerical models for structural failure of laminated composites. Using thin-ply pre-pregs, this paper demonstrates that this translaminar toughness is not an absolute, but rather in-situ, property and depends strongly on the 0° ply-block thickness, even in situations where delamination and diffuse damage are inhibited. We used two different grades of a thin-ply carbon-epoxy system to produce four different 0° ply-block thicknesses ranging from 0.03 mm to 0.12 mm, and measured the respective translaminar fracture toughness using compact tension tests. SEM and X-ray analysis showed no delamination nor diffuse damage. Yet, the translaminar fracture toughness increased from 46 to 104 kJ/m2 (initiation), and from 49 to 160 kJ/m2 (propagation), for the thickness range above. This finding has significant implications for the development and use of ply-level numerical failure models, for structural design with thin-ply composites, and for the development of thin-ply material systems.