Six sediment samples were collected from the northern Arabian Gulf . Nickel was added to each sediment-seawater suspension and the concentration of total dissolved Ni in the suspensions was monitored for 75 days . The analytical data were used to obtain a linear regression equation relating Ni" activity in the sediment suspensions to pH . Using this equation and thermodynamic information, the distribution of Ni species in the suspensions was calculated . The major inorganic species, extrapolated to 35%. salinity and pH 8 .1, were : Nit + (60.1%), NiCl + (16 .9%), NiCl (5 .0%), NiOH + (0 .4%), and NiSOO (17 .5%) . An increase in the seawater salinity increased the concentration of total dissolved Ni and Ni chloro-complexes . A decrease in pH of seawater increased total dissolved Ni and decreased NiOH + complex, but the proportion of other species remained unchanged . There was no significant (P < 0.05) effect of Cu, Cd, Pb, Fe, Mn, and Al additions on Ni sorption in the marine sediment suspensions .