Abstract The effects of humic substances on the interaction of hydroxy-Al ions and orthosilicic acid at a Si concentration of 1.6 x 10 −3 M, a Si/Al molar ratio of 1.0, an OH/Al molar ratio of 3.0, and humic or fulvic acid concentrations of 1 to 1000 mg/liter were studied. Humic substances at concentrations > 100–300 mg/liter significantly perturbed this interaction and thus inhibited the formation of allophane. The Na-pyrophosphate- and Na 2 CO 3 -soluble fractions of the precipitates (>0.01 μm), formed in the presence of the humic substances at a concentration of 300 mg/liter, were, respectively, 71% and 22% in the humic acid system and 17–24% and 36–40% in the fulvic acid systems. Selective dissolution data and differential infrared spectroscopy showed that “proto-imogolite” allophane was a major component in the pyrophosphate- and Na 2 CO 3 -insoluble fraction of the precipitates formed in the presence of fulvic acid. The pyrophosphate- and Na 2 CO 3 -insoluble fraction of the precipitates formed in the presence of humic acid showed a very weak infrared spectrum. The present results show that humic substances are pedogenically significant in impeding the formation of allophane.