Equilibrium data at (293 +/- 2) K are presented for benzoic acid in water and three different organic phases (heptane, methylbenzene, and trichloroethylene). The monomeric partition constant, K(P)(MON), and the dimerization constant, K(D), for the organic phase were determined at 293 K. For the heptane/water benzoic acid system, K(P)(MON) equals (0.209 +/- 0.049) m(AQ)(3)center dot m(ORG)(-3), and K(D) equals (1.33 +/- 0.68) m(3).mol(-1). For the methylbenzene/water benzoic acid system, K(P)(MON) equals (1.17 +/- 0.92) m(AQ)(3)center dot m(ORG)(-3), and K(D) equals (0.39 +/- 0.64) m(3). mol(-1). For the trichloroethylene/water benzoic acid system, KK(P)(MON) equals (0.808 +/- 0.295) m(AQ)(3).m(ORG)(-3), and K(D) equals (1.18 +/- 0.89) m(3).mol(-1). Equilibrium constants were obtained from UV vis spectroscopy and conductivity experiments. The equilibrium data of these extraction systems can be used to quantify liquid-liquid mass transfer rates with extraction systems that require low flammability risk (trichloroethylene), low costs (methylbenzene), or setups constructed from polymethyl methacrylate (heptane).