The stability of arsenic species in environmental samples during sampling, storage and processing is crucial for speciation studies. Losses, interconversion and degradation of arsenocompounds can occur during these steps as a result of the interaction with the container material, microbial activity and temperature or light. In order to study the possible effects of storage and processing on arsenic compounds, subsamples of alga Cystoseira mediterranea Sauvageau were stored under different conditions (nonfrozen; frozen at 18 C for 24 h and 45 days; frozen at 80 C for 24 h and 45 days) and subjected to one of the following processing methods: chopping into fine pieces with a knife; grinding by hand in a glass mortar under N2(l); drying under an air current at room temperature (25 C); drying in an oven at 40 C; lyophilization. The total contents of arsenic and arsenic compounds were then determined in these subsamples. Non-frozen subsamples were of 150 mg As per kg while frozen-defrosted samples were of 92 mg As per kg. Nevertheless, in spite of this loss in content, the proportion of arsenic compounds was maintained in both cases. Our results indicate that freezing is an unsuitable storage method for macroalgae because losses of arsenic may occur. We recommend a drying method for this purpose.