From a survey of the literature on the toxicity of ammonium salts, phenol, cyanide and sulphide to rainbow trout, and from determinations of the toxicity of sodium thiocyanate and sodium thiosulphate, it is postulated that the toxicity of spent still liquors from the distillation of coal should be due mainly to their content of ammonia and monohydric phenols. This is confirmed by experiments showing that the toxicity of an equivalent mixture of ammonium chloride and phenol is nearly as great as that of a spent liquor from a gas works, and that phenol is almost as toxic as mixtures of the monohydric phenols known to be present in such liquors.

Experiments on the effect of pH value, hardness, dissolved-oxygen concentration and temperature on the threshold concentration of monohydric phenols are described and compared with similar data for ammonia.

Experiments with ammonia and phenols suggest that a mixture of these substances is at its threshold concentration when AS/AT+PS/PT= 1, AS and PS being the concentrations of un-ionized ammonia and monohydric phenols in solution and AT and PT being the threshold concentrations of these substances when tested individually in the same dilution water. The effect of dissolved-oxygen concentration on the toxicity of an ammonia-phenol mixture is examined and compared with the effect of this variable on the individual toxicities of ammonia and phenols.

A method based on these experiments for predicting the toxicity of ammonia-phenol mixtures from the chemical composition of their solutions is described, and evaluated against laboratory determinations of the toxicity of spent liquors from a coke oven, and against the death or survival of trout held captive in a stream polluted with spent liquor from a gas works. It is concluded that the correspondence between the predicted and observed toxicities is good enough for the method to be used as a basis for assessing whether trout could live in a stream to which a particular spent still liquor was discharged, or when deciding what treatment the effluent should receive to make it safe for such fish after discharge.