The solubilization phenomenon was investigated in mixed surfactant systems. The solubilization power of a mixed surfactant reaches its maximum at a particular temperature at each mixing ratio of surfactants. When the mole fraction of C4E1 in the total surfactant (w(1) value) was varied in a water/C12E5/C4E1/decane system, the minimum mole fraction of total surfactant in the system necessary to obtain a single microemulsion phase (ξ value) was almost unchanged for w(1) < 0.3, whereas it increased remarkably for w(1) > 0.8. The molar solubilization capacity (C-s = (1 - ξ)/ξ) of the mixed surfactant decreased remarkably for w(1) < 0.3, whereas it decreased gradually for w(1) > 0.8. The result &VERBAR; dξ/dw(1)&VERBAR; (w1 < 0.3) < &VERBAR; dξ/dw(1)&VERBAR; (w1 > 0.8) is due largely to the characteristic of the function ξ(C-s) = 1/(1 + C-s), specifically, &VERBAR; dξ/dC(s)&VERBAR;(w1 < 0.3) < &VERBAR; dξ/dC(s)&VERBAR;(w1) > 0.8, where dξ/dw(1) = (dξ/dC(s))(dC(s)/dw(1)). The partial molar solubilization capacity ((C) over bar (s)) of C4E1 was negative at almost all w(1), but the (C) over bar (s) value of C12E5 went through a maximum on the addition of C4E1. Propanol (a cosurfactant) has the same effect on the solubilization phenomenon in the water/C12E6/propanol/heptane system. In the water/C12E5/C12E7/decane system, the (C) over bar (s) value of each surfactant did not vary greatly as the mixing ratio of surfactants was varied. The C-s and ξ values were close to molar additivity for each mixing ration. © 2005 Elsevier Inc. All rights reserved.