A phytochemical study on the lichen Dermatocarpon luridum led to the isolation of four known compounds, including mycosporine glutaminol (1), 2-amino-3-acetylaminopropionic acid (2), (22E,24R)-ergosta-7,22-diene-3,5,6-triol (3) and (2S,3S,4R,2’R)-2-(2’-hydroxytetracosanoylamino)octadecan-1,3,4-triol (4). Their structures were determined by extensive spectroscopic analyses including UV, IR, ESI-HRMS, 1D, 2D NMR and specific rotation as well as by comparison of the data with those in the literature. The pKa value in an aqueous solution of (1) was determined by UV-VIS spectrophotometry at 298 K. With the low pKa value 1.29, only the neutral form of (1) was present and responsible for the UV absorption in water at pH 7.