The phase behavior of C10E4-oil-water systems at constant o/w ratio and variable temperature (fish dia.-ram) has been investigated for several homologous oil families. The temperature T* and surfactant concentration C* at the critical point were determined for 10 n-alkanes varying from C-6 to C-28 as well as for a series of alkylcyclohexanes and alkylbenzenes. On the basis of T*, equivalent alkane carbon numbers (EACN) were assigned to nonlinear alkanes, alkylbenzenes, and alkylcyclohexanes. The consistency of the method was shown by corroborating that the EACN values of oils previously investigated with other CiEj (dibutyl ether, squalane, isopropyl myristate, and dodecylbenzene) are the same when determined with C10E4. The fact that two oils of different nature but with the same EACN (i.e., the same T*) do not exhibit the same C* is discussed in terms of monomeric solubility of the surfactant in the oil (CMCoil). (c) 2006 Elsevier Inc. All rights reserved.