The effects of controlled release of two inhibitors (2,4‐dinitrophenol (DNP) or benzoic acid) from simple vinyl copolymer films on biofilm formation and activity were evaluated using a consortium of marine bacteria in a challenge assay. DNP is an uncoupler of membrane proton gradients. Benzoic acid is a repellant. The batch assays were conducedin vitro. Acriflavine direct cell counts (AFDC), iodonitrophenol (INT) tetrazolium dye reduction activity, presence of division septa, and biofilm DNA were used as response variables. DNP release at high release rates (15 to 70 μg·cm−2d−1) was inhibitory to cell attachment and activity. DNP release at low release rates (0·5 to 40 μg·cm−2·d−1) was inhibitory to biofilm activity. Benzoate release inhibited biofilm activity at high release rates (10 to 60 μg cm−2·d−1) and stimulated activity at low release rates (0·5 to 20 μg·cm−2·d−1). INT reduction, presence of division septa, and biofilm DNA content were more sensitive response variables than AFDC in assessing cause and effect. These data suggest that the controlled release of inhibitors from films can be used to evaluate both beneficial and detrimental effects on biofilm formation. Certain modes of inhibition (e.g. uncoupling) are more effective in controlling biofilm development.