Saxitoxin (STX) is the most toxic non-protein substance known. STX-producing cyanobacteria have been identified in most continents and are being detected more widely because of global warming, threatening human drinking water supplies worldwide. Removal of these components can be accomplished by adsorption on granular-activated carbon (GAC) but little is yet known about the kinetics of the process. This research investigated adsorption kinetics and diffusion behaviour of a decarbomoyl saxitoxin (dc-STX) and a carbamate toxin (STX) on four coconut shell-based GAC samples with different pore size distribution. It was observed that equilibrium concentration was reached within 24 h and that a pseudo-second-order model best represented experimental data. Of the four GAC samples tested, the example with the largest volume of mesopores adsorbed more STX and with a faster upload rate, while dc-STX was adsorbed equally in all four GAC samples.