Microscope tests to analyze degradation of wool and wool blends after high-temperature dyeing (120°C) are reviewed. To avoid heavy damage to the wool fiber, formaldehyde is used to react with the wool and alter its chemical behavior, making it less sensitive to high dyeing temperatures and other chemical influences. This means that most chemical degradation tests can no longer be used and therefore criteria for distinguishing between wool with and without formaldehyde treatment are investigated. -- AATA