Aims: To determine the effect of chlorine on mixed bacterial biofilms on stainless steel (SS) and conveyor belt surfaces. Methods and Results: Biofilms were exposed to pH-adjusted (6·5) and non-pH-adjusted solutions of chlorine (200, 400 and 600 ppm) for either 2, 10 or 20 min and survivors enumerated. There were significant differences in cell death relating to chlorine concentration and exposure time for the cells attached to the SS, with solutions adjusted to pH 6·5 being more effective at reducing numbers. In contrast, on conveyor belt surfaces cell numbers decreased by less than two logs after 20 min regardless of treatment. Conclusions: Chlorine effectiveness is dependent on its concentration, solution pH, exposure time, the nature of the surface and the microbial species present. Significance and Impact of Study: In the interests of food safety it is important that sanitizer users are aware of the conditions that effect their performance.