Circular dichroism (CD) is presented as a reliable and sensitive method of determining the diadic frequency composition of alginate (F(GG), F(MM) and F(GM+MG)). The availability of samples, very largely or even completely conforming to the limiting structures of polymannuronate (MM)(n), polyguluronate (GG)(n) and polyalternating MG (MG)(n), respectively, allowed the limiting CD spectra for each alginate diad to be obtained. These showed very different CD behaviour, thus pointing out the crucial importance of the neighbouring residue in chiroptical properties. Using an iterative best-fit procedure, the diadic composition of commercial alginates could be obtained from their respective CD spectra by means of a linear combination of the spectra of the three limiting diads. The results were found in excellent agreement with the composition parameters obtained by 1H NMR spectroscopy.