Abstract In this work were studied IR spectra of the type of sulfated polysaccharides as agar of Phyllophora nervosa and various agars and carrageenans. Agars used were from Phyllophora nervosa, Gracilaria verrucosa, Pterocladia capillecea, Ceramium rubrum, Hypnea musciformis, agar (Pasteur and commercial) and carrageenans were K -, tw, type LC and carrageenan obtained from Grateloupia dichotoma, Acanthophora delilei. IR spectra of these agars were compared with those of carrageenans. The bands of 805 and 705 cm-1 characteristic for carrageenans were absent at the spectra of the agars tested. Agars showed the same bands at (736, 771, 927, 968, 1253cm-1). It was proved that the sulfated polysaccharide obtained from Phyllophora nervosa was agar.