The seasonal sunlight-induced photodegradation of seven emerging contaminants (ECs) in a municipal wastewater treatment lagoon was examined in a lab-scale study over a year. There were obvious seasonal trends for the degradation rates of all the ECs in both ultrapure water and lagoon wastewater matrices with strong correlations (R² = 0.65–0.90) between sunlight irradiance and their degradation rates in pure water, indicating the dominant influence of natural sunlight intensity. This suggests the importance of recording light source conditions for the purposes of data interpretation and modelling of sunlight-induced photolysis. It was demonstrated that photolysis of these ECs was likely to occur only within the first 10 cm of the lagoon surface. Strongly temperature dependent photodegradation was observed for the five ECs which primarily underwent indirect photolysis in the lagoon wastewater, and activation energy values (which could be used for simulation of the photodegradation process) were obtained for these compounds. The quantum yields of the ECs were determined under wastewater-relevant pH conditions. The APEX environmental model was adjusted to simulate the photodegradation of these ECs in the lagoon wastewater. The predicted results were largely in agreement with the seasonal measurements, with some overestimated photodegradation rates obtained for winter, suggesting that temperature should be included as a factor for modelling the photolysis of ECs. These results indicate that there is potential to establish a model to predict the photodegradation of a wide range of ECs in wastewater treatment lagoons to facilitate risk assessment of the treated wastewater.