Mainly because of cost-saving and environmental concerns, ozone has been introduced as a cleaning agent in semiconductor manufacturing in recent years. This work reports the determination of the most important degradation products formed after the stripping of two different types of photoresist (PR) (I-Line and DUV resist) by ozone processing. Six low molecular weight carboxylic acids could be identified for the I-line PR, five for the DUV PR. The behavior of these degradation products under continuous ozonation as well as their stability in water was also investigated. Despite the chemical similarity of these carboxylic acids, their behavior appears to be quite different.