The long‐term degradation of Corprene‐backed hydrophones has been investigated by the use of color microphotography using finegrain film with slide‐projected magnifications up to 500 × life size. Oil absorption is significant and visually obvious when compared to pure cork and non‐oil‐soaked Corprene. Variations in cork to neoprene ratio and grain of various samples of the same military specifications are visually apparent and their effect on oil absorption is discussed.