Both the ability of membrane filters to retain iron added (as both Fe(II) and Fe(III)) to surface waters from various sources, and the ability of near ultraviolet light to induce dissolution of colloidal iron oxides suspended in waters from different locations are examined here. In brief, the presence of naturally occurring organic matter exerts a dominating effect on the tendency of iron oxide particles to interact with each other (both in suspension and on or within the membrane matrix). In coastal waters in the vicinity of the Orinoco River, growth of iron oxide particles initiated on addition of either Fe(II) or Fe(III) to the sample of interest, is significantly retarded in comparison to iron particle growth in open ocean waters. Near-UV light appears to retard particle growth following both Fe(II) and Fe(III) addition to a coastal sample but does little to induce the dissolution of colloidal oxides. ¿ American Geophysical Union 1993