In the presence of millimolar concentrations of inorganic phosphate, native Spirulina maxima glutathione reductase (NAD[P]H:GSSG oxidoreductase EC 220.127.116.11.) changes its aggregation state. The oligomeric structure of the enzyme was notably dependent upon phosphate molarity, ranging from a dimer-tetramer equilibrium at relatively low phosphate concentrations into a tetramer-octamer equilibrium at moderate or high phosphate concentrations. In spite of the changes in quaternary structure, the tetramer remains as the most stable and abundant species. Sodium chloride solutions were not able to produce a similar effect, thus discarding an unspecific ionic strength effect.