The reactive extractions of formic acid with tri-n-octylamine (TOA) dissolved in three solvents with different dielectric constants (dichloromethane, butyl acetate, n-heptane) without and with 1-octanol as phase modifier were comparatively analyzed. The results indicated that the mechanism of the interfacial reaction between acid and extractant (Q) is controlled by the organic phase polarity. In the absence of 1-octanol, the structures of the extracted complexes are (HA)(2)Q(2) for dichloromethane and butyl acetate, and (HA)(2)Q(4) for n-heptane. These structures are modified by adding 1-octanol and become (HA)(2)Q for extraction in dichloromethane or butyl acetate, and (HA)(2)Q(2) for extraction in n-heptane. Although the presence of 1-octanol improves the extraction efficiency, it leads to a reduction of the extraction constants for all considered solvents, an influence that is more significant for n-heptane.