AbstractAlgal particles fromLaminaria hyperboreastipes andLaminaria digitatawere exposed to a stream of hydrochloric acid in a continuous flow apparatus, and the ion‐exchange process Ca/H was investigated. The exchange rate was shown to depend upon such factors as acid concentration, particle size, agitation and time of acid residence.The results show that the exchange rate followed a first‐order reaction in the conditions used, and the exchange velocity was proportional to the logarithm of the hydrochloric acid concentration. Evidence is given which indicates that the exchange process is governed by a film‐diffusion mechanism, and the diffusion of calcium across a thin liquid film enveloping the particle is suggested to be the rate‐determining step. Exchange experiments with particles fromAscophyllum nodosumshow that another rate mechanism has to be considered in th