Controlled laboratory (4°C) experiments were undertaken of barite precipitation and dissolution in natural (filtered and unfiltered samples from Esthwaite Water, UK) and synthetic (with and without added humic and fulvic acids) lake waters. The ion activity products (IAP = {Ba2+}{SO4 2-}) for the dissolution and precipitation experiments in synthetic lake waters, with no added humic substances (HS), and for the dissolution experiment in the filtered natural water reached a common equilibrium -log10 IAP value of 9.67 ± 0.04 (n=10). The IAP values for the precipitation experiments in filtered and unfiltered natural water and in synthetic lake water, with added HS, indicated that the solutions remained highly supersaturated throughout. The degree of supersaturation was the same irrespective of the nature of the HS (fulvic or humic acid) and its concentration (1 or 5 mg/L). The two contrasting saturation states observed overall in the precipitation experiments were reflected in the nature of the resulting barite phases. In synthetic lake water with no added HS, the barite was present as large (7-15 μm) euhedral crystals, whereas in filtered natural water and synthetic water with added HS, particles were far less abundant and occurred exclusively as discrete 1-μm spherulites. The observed state of supersaturation in the experiments in natural water and in synthetic lake water with added HS is attributed to crystal-growth inhibition, due to adsorption of HS to the spherulite surfaces. Our results support the view that natural barite precipitation in Esthwaite Water occurs through direct biological mediation. The results also indicate that dissolved HS will generally inhibit extracellular barite precipitation in (supersaturated) freshwater systems.