Dried and reconstituted mud from pairs of primitive, medium, productive, and acid bog lakes was shaken to phosphorus equilibrium with water. Minimal P was released at pH 5.5–6.5, and ranged from scarcely above the level of chemical detection up to 0.2 ppm in productive lakes. Further acidity caused a slight increase of P in the water, up to 0.3 ppm, and alkalinity a larger increase, up to 0.5 ppm. At all pH levels the quantity released increased in the order of lake types stated above. The pH versus P curve for whole mud was shallow or saucer‐shaped; ashed mud gave a similar but deeper or cup‐shaped curve. Thus a productive lake ash at pH 4 released 1.0 ppm and at pH 8, 1.2 ppm. When 1 ppm of P was added to the water prior to the shaking, the lake type reacted differently. Acid bog and productive lake muds left the added P in the water, while unproductive lake muds removed most of it under acid conditions but not at pH 7 or more. The adsorption behavior of ash is similar to that of Bentonite, Fuller’s earth, or ferric hydroxide gel.