Co-extruded core-membrane formulations have been extensively explored as long-acting subcutaneous implants or vaginal inserts, guaranteeing zero-order release. Although the membrane thickness and homogeneity determine the final drug release, these parameters are still evaluated offline via destructive testing. The present study aims at tackling this shortcoming by introducing an in-line, non-invasive, fast and reproducible methodology based on optical coherence tomography (OCT) to evaluate the membrane characteristics of ethylene vinyl acetate (EVA) based co-extrudates. The novel setup enables a membrane representation and thickness analysis in real time during the manufacturing process, while the characterized membrane thicknesses are in good agreement with those obtained by optical microscopy. In addition, OCT offers novel insights into essential membrane characteristics that have not been visualized before. A case in point is the visualization of potential membrane defects, e.g. delaminated membranes, rough surfaces or viscosity mismatches in the core-membrane interface. Due to the obtained wealth of information combined with the instantaneous and destruction-free measurement, OCT has the potential to become the process analytical technology of choice for co-extruded dosage forms.