Abiotic interactions occurring between dissolved humic substances, iron, and phosphate were investigated by addition of radioisotope tracers (55FeCl3 and PO43−) to surface water samples collected from catchments subjected to contrasting pH manipulations. Increases from pH 4 to circumneutral pH due to catchment liming resulted in increases in the proportions of added ionic 55Fe and 32P recovered in higher molecular size fractions following gel chromatography. Changes in ionic composition due to catchment liming did not result in marked changes in redistributions of added 55Fe and 32P. Comparatively small differences between the pH values and ionic compositions of the control and artificially acidified subcatchments of a humic lake did not result in substantial changes in the redistributions of added 55Fe and 32P.