Mecoprop is a common pollutant in effluent-, storm- and groundwater as well as in leachates from derelict dumpsites. Thus, bioremediation approaches may be considered. We conducted batch experiments with MBBR-carriers to understand mecoprop’s degradation process. As a model, the carriers were incubated in effluent-wastewater containing mecoprop (10, 50 and 100 µg L^-1; n=3). The degradation of mecoprop (10 µg L^-1) was also tested with methanol co-spiking to assess co-metabolic processes. Besides, we quantified mineralization with a parallel batch experiment. The biofilm microbial communities were investigated for changes in composition as a response to mecoprop concentration and transformation, as well as screened for catabolic genes through metagenomic sequencing. Initial mecoprop concentration and accumulated mecoprop degradation impacted the microbial communities. The removal of (S)-mecoprop prevailed over the (R)-mecoprop one. This was associated with microbial composition, where several operational taxonomic units (OTUs) co-varied positively with (S)-mecoprop removal. The removal-rate constant of (S)-mecoprop was 0.5 d^-1 in the 10 µg L^-1 set-up but it decreased in the 50 and 100 µg L^-1 set-ups. The addition of methanol prolonged the removal of (R)-mecoprop. During mecoprop degradation, 4-chloro-2-methylphenol was formed and degraded. A new metabolite (4-chloro-2-methylphenol sulphate) was identified and quantified.