N-Octadecylamide of alginic acid has the potential to become an interesting substance with benefits for human health as a sorbent of cholesterol and bile acids. This work is focused on detailed characterization of alginic acid and product of amidation reaction with n-octadecylamine and mainly extends knowledge of reaction products. Different methods were used for characterization of whole reaction from starting material alginic acid to product its amidation.