In the present work, the influence of the fibre treatment on the transverse stiffness of glass/epoxy unidirectional laminates is discussed. The transverse modulus of UD-plates with different fibre treatments was determined by the transverse tension test and by vibration tests with holography and an identification procedure. It will be shown that a significant influence of the fibre treatment (interphase) can be observed if the transverse modulus is determined by tension tests. If the transverse modulus is determined by non-destructive testing with vibration tests, including a mathematical identification procedure, no significant influence of the fibre treatment can be observed. This indicates that the results of transverse tension tests should be used very carfully to describe the influence of the interphase on the transverse modulus of unidirectional reinforced plates. It is recommended to use some additional (non-destructive) testing, e.g. vibration tests.