On-line particle count and turbidity data was used to monitor the impacts of transient particulate water quality events on the inactivation of indigenous total coliform bacteria by a pilot-scale ultraviolet (UV) disinfection unit at two drinking water treatment plants (referred to as WTPs ‘A’ and ‘B’). The intent of the study was to assess the performance of UV disinfection when exposed to temporary ‘worst case’ particulate water quality conditions in a drinking water treatment context. The pilot unit was installed downstream of a poorly operating filter at WTP ‘A’ and downstream of an intentionally destabilized Actiflo® system at WTP ‘B’. In each case, elevated particle content (turbidity up to 2.9 NTU) did not cause observable adverse effects on the UV disinfection of total coliform bacteria over the test periods, even when a particle extraction technique was applied to count particle-associated bacteria in the water samples.