Palavras-chave: mistura em tanque, floculação, precipitação, formulação.ABSTRACT -The objective of this study was to evaluate the physical interaction between simulated tank mixtures of different classes of pesticides. The experiment was conducted at the Chemistry Laboratory of the University of Mato Grosso, Brazil, from June to August 2010. The treatments consisted of mixtures of six herbicides (glyphosate SC, glyphosate WG, lactofen CE, fomesafen SC, haloxifop-R CE, and fluazifop-p-butyl EW) with six types of insecticide (methomyl CS, clorpirifos CE, teflubenzuron SC, triflumuron SC, cipermetrin CE, and tiametoxam + lambda-cialotrin SC) in the absence and presence of two pH reducers (pyroligneous acid and boric acid) with four replications. A scale from 1 to 5 was used to evaluate the degree of mismatch, where 1 is immediate separation of the mixture (which is recommended not to apply), and 5 is homogeneity of the mixtures. The major physical incompatibilities in the herbicide and insecticide mixtures were observed in the presence of glyphosate WG and lactofen SC. Pyroligneous acid and boric acid showed to be good alternatives as pH reducers in the preparation of sprays using the herbicide and insecticide mixtures. Tank mixtures of glyphosate formulation SC, clorpirifos CE, and lactofen CE + clorpirifos CE must be avoided.