Amine-aldehyde resins are disclosed for removing a wide variety of solids and/or ionic species from the liquids in which they are suspended and/or dissolved. These resins are especially useful as froth flotation depressants, for example in the beneficiation of value materials (e.g., bitumen, coal, or kaolin clay) to remove impurities such as sand. The resins are also useful for treating aqueous liquid suspensions to remove solid particulates, as well as for removing metallic ions in the purification of water.