Algae are photosynthetic autotrophic organisms that inhabit virtually all habitats. They are divided into microalgae and macroalgae. According to the pigment they contain, they are divided into 3 groups: brown (Phaeophyta), green (Chlorophyta) and red (Rhodophyta) algae. Brown algae live exclusively in the seas, they are most often attached to rock surfaces, rocks or other organisms. The brown pigment fucoxanthin, which masks chlorophyll and other carotenoids, gives them their characteristic brown colour. The brown macroalga Cladostephus spongiosus (Hudson) C. Agardh 1817 is widely distributed in the Mediterranean Sea and it could be found in the Atlantic coasts from Morocco to Ireland. The analysis was performed on a fresh and dry sample of C. spongiosus alga sampled in May (at a sea temperature of 20.01 °C) and August (at a sea temperature of 28.01 °C). The coupled gas chromatography system with mass spectrometry was used for the volatile hydrodistillate compounds analysis. By comparing the obtained results, differences in the abundance of terpenes, chlorophyll derivatives, fatty acids and derivatives and aliphatic saturated and unsaturated compounds were observed with respect to the month of sampling. In the sample of C. spongiosus from May, terpenes are most represented (48.70%, fresh; 38.05%, dry), while in the sample from August their content remarkably decreases (12.57%, fresh; 17.20%, dry). The difference was also observed in the presence of chlorophyll derivatives whose abundance in the sample from August (16.07%, fresh; 19.37%, dry) was almost three times higher than the abundance of the same group of compounds in the sample from May (5.29%, fresh; 7.36%, dry). On the basis of the conducted research, it can be concluded that the sea temperature affects the chemical composition of the alga C. spongiosus.