Proceeding in a homologous series of oils and nonionic amphiphiles, we study the dependence of the coordinates of tricritical points and wetting --> nonwetting transitions on the carbon number of the oils, and the strength of the amphiphiles. We find an inverse relation in that weakly nonpolar oils require strong amphiphiles for to be emulsified, whereas for emulsifying strongly nonpolar oils, weak amphiphiles suffice. The results are discussed qualitatively on the basis of a simple model inspired by Harkins.