The occurrence and formation of chlorinated and brominated trihalomethanes (THMs) were studied in artificial saltwater and natural seawater marine aquaria as well as in groundwater and surface waters. a new headspace solid-phase microextraction method was used, which included gas chromatography-mass spectrometry and provided method detection limits of less than 0.1 mg/L, with a signal-to-noise ratio of 3 to 5. the results showed that disinfection by using either chlorine or ozone caused a significant formation of THMs in situ. Speciation of the THMs was a strong function of the water matrix, with initial bromide concentrations playing a pivotal role. These results provide a tool for understanding and monitoring the formation of key disinfection byproducts in marine aquaria that may cause respiratory, eye irritation, or other health concerns.