Different sodium alginates derived from Laminaria hyperborea, with various mannuronate/guluronate (M/G) ratios and molecular weights, were immersed in calcium chloride solution and converted to mixed sodium/calcium salts. Thin films were obtained and studied using Fourier transform infrared spectroscopy (FTi.r.). FTi.r. spectra were recorded as a function of time and the ion exchange between sodium and calcium monitored. Peak shifts, difference in peak shapes and the appearance of new peaks were observed, and some explanations of the phenomena observed are proposed. In parallel, the samples were analysed quantitatively by atomic absorption/emission spectroscopy. From these results, peak wavenumbers derived from the FTi.r. spectra could be correlated quantitatively to the sodium and calcium content.