Duckweeds—a group of floating leaf macrophytes from the family of Lemnaceae—have become a major area of interest in the fields of basic and applied aquatic sciences in recent decades, including their use in water purification. Aiming to fulfill one of the gaps in the role of light intensity in duckweed efficiency in organic matter removal, we carried out a laboratory experiment with the use of two duckweed species: Lemna minor and Spirodela polyrhiza. Our main finding was that the intensity of light has a positive effect on the process of water purification from organic compounds by Lemna minor. However, this was not applicable to Spirodela polyrhiza due to the fact that the growth of the species was inhibited by high light intensities.