This dataset contains raw and processed data for the manuscript: "Temporal variations of microplastic particle concentrations across stages of a wastewater treatment plant" TO BE COMPLETED: DOI, Journal Info etc. will follow when published Please cite this article when using the data. The contribution of wastewater treatment plants (WWTPs) to microplastic pollution in surface waters remains controversial. Estimates of microplastic particle (MP) loads discharged from WWTP are mostly derived from grab sampling campaigns, neglecting potential temporal variations in effluent MP concentrations. We therefore investigated daily, weekly, and seasonal variability of MP concentrations across a modern WWTP (activated sludge treatment complemented with ozonation and sand filtration as tertiary treatment). We employed a novel quantum cascade laser based – infrared imaging technology that increased sample throughput by more than an order of magnitude compared to conventional Fourier transform infrared systems and allowed an automatic detection of spiked internal standards. MP concentrations in sludge and effluent samples varied within one order of magnitude, without clear temporal patterns throughout the year. Median number and mass concentrations of MP were similar across all sampling points and matrices and ranged between 5·103 and 1·104 # gTSS-1 (0.5 and 2 mg gTSS-1). Based on the absence of temporal patterns, we estimate annual MP per capita emissions to 7·106 to 9·106 # (cap·year)-1 (0.7 to 1.3 g (cap year)-1) for WWTPs employing an activated sludge treatment. The tertiary treatment reduces the MP loads by an additional 2 log units, resulting in estimated annual MP per capita emissions of 5·104 to 1·105 # (cap·year)-1 (0.005-0.01 g (cap·year)-1). The obtained emission rates can serve as guidance for estimating the share of MP discharged from WWTPs into surface waters once monitoring data on the MP loads in surface waters become available.