Humic substances (HS) in naturally acidic waterways have been suggested to provide protection to aquatic organisms exposed to low pH conditions. Despite this, little is known about the ability of HS to increase survivorship of freshwater organisms as pH decreases in waters of varying hardness. This study explored the ability of HS in the form of Aldrich humic acid (AHA) to increase survivorship of the freshwater shrimp (Caridina sp. D) at low pH in artificial soft (represent-ative of naturally acidic environments) and hard waters (artificial and natural Dee River water). Freshwater shrimp were exposed to pH treatments ranging from pH 7 to pH 3.5, with and without 10 or 20 mg/L HS treatments. In low pH water, shrimp mortality was higher in artificial hard water (LC50 at pH 4.95) and natural hard water (LC50 at pH 4.74), compared with soft water (LC50 at pH 4.27). HS substantially decreased the threshold at which pH caused 50% mortality to the freshwater shrimp, with that threshold shifting from 4.95 to 4.47 in artificial hard water, from 4.74 to 4.50 in natural hard water and from 4.27 to 4.18 in soft water. The results of this study are valuable in contributing to an improved understanding of the influence HS and water hardness has on the toxicity of low pH to freshwater biota and may have important implications for the management of acidified waterways.