Concerns regarding specimen integrity have long been a major issue of urine drug testing due to acts of urine adulteration. At a high concentration, in vitro urine adulteration using sodium hypochlorite (bleach) produced false-negative results for 3,4-methylenedioxymethamphetamine (MDMA) in CEDIA^® immunoassay screening with strong negative readings. However, these strong negative readings may act as a warning sign for further investigation of the sample where the detection of a unique marker in the form of N-chloroMDMA will suggest urine adulteration via bleach. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) identified N-chloroMDMA is a major product formed between hypochlorite and MDMA in urine. N-ChloroMDMA was found stable at 4 °C for at least 10 h, but decomposed over time at room temperature (20 °C) with MDMA being identified as one of its main decomposition products.