Adulteration is an illegal practice often committed in food and cosmetic industries, yet rarely in environmental regulation compliance. The Chinese Ministry of Ecology and Environment recently issued a law prohibiting any intentional use of ‘interfering agents’ during wastewater compliance testing. For the first time, such illegal practices are brought to public attention, though they have been barely discussed previously in the scientific literature. The fraudulent method involves the addition into wastewater of a sodium chlorate-based compound, named 'COD remover,' which artificially decreases the chemical oxygen demand (COD) measured during legal testing using the dichromate method. Such practices can decrease COD from 214 mg/L to below 100 mg/L. COD removers are designed to work under testing conditions, and they do not effectively purify wastewater. The use of COD removers has created an underground industry, where users can anonymously purchase those products in 25-kg packages from all major domestic online retailers in China. A notable case was recently publicized after the new legislation, where 131 tons of COD remover had been used in a municipal wastewater treatment plant. Nonetheless, very little is known about other possible cases in China and elsewhere, and whether COD is the sole target of fraudulent practice in wastewater testing. For instance, nitrites could be used as interfering agents in the iodometric method for measuring dissolved oxygen in water and wastewater. There are also reports of chemical adulteration in online monitoring of gas emissions. Since targeted chemical adulteration is based on the knowledge of reactions and mechanisms of testing methods, there is a need to develop more robust and adulteration-proof methods for wastewater testing.