A large fraction of antibacterial agents administered to farmed fish is released into the environment, either in leaching from uneaten medicated pelleted feed, or through urinary, branchial and faecal excretions. Processes such as hydrolysis and photolysis might be important for the elimination of antibacterial agents from water. The aim of the present study was to compare hydrolysis and photolysis of oxytetracycline (OTC), oxolinic acid (OXA), flumequine (FLU) and florfenicol (FLO) in three types of water (deionised water, freshwater and seawater) under abiotic conditions. For each antibacterial agent, a 7-day factorial experiment was designed in order to study the effect of light, pH and dissolved oxygen on hydrolysis and photolysis in deionised water. Furthermore, 14-day kinetics experiments were performed for each antibacterial agent and each type of water. FLO was not degraded by hydrolysis or photolysis phenomena. Hydrolysis phenomena only concerned OTC and were responsible for about 20% of its degradation following a 14-day exposure at a temperature of + 8 °C. Photolysis was responsible for about 70% of the OTC degradation in freshwater and seawater and 10% of the OXA and FLU degradation in seawater following the same exposure. Water pH played a role in OTC hydrolysis and photolysis. There was no close connection between OTC hydrolysis and photolysis and OTC binding to cations and organic matter, but it would be suggested that there was a connection between absence of OXA and FLU photolysis and OXA and FLU binding to organic matter.