A single-phase, either isotropic or anisotropic formulation is disclosed, which contains perfluorinated oil and one or more surfactant(s), is useful as an excipient for cosmetic and dermatologic formulations and is characterized in that at least one surfactant is selected from those having the following general formula: ##STR1## wherein: R.sub.F is an F--(CF.sub.2 --CF.sub.2).sub.i -- group;X is a group selected from: ##STR2## --(CH.sub.2 --O).sub.K --; and --(CH.sub.2 --S).sub.K --; wherein R.sup.I is a (C.sub.1 -C.sub.4)-alkyl;Y is a group selected from: NH.sub.2 (R.sup.II).sub.2 and N(R.sup.II).sub.4 ; wherein R.sup.II is a hydrogen atom, or a group selected from --CH.sub.2 --CH.sub.2 --OH and (C.sub.1 -C.sub.4)-alkyl;i is an integer comprised within the range of from 3 to 8;i is zero or 1;K is 2.l;l is an integer comprised within the range of from 1 to 10;m is 1 or 2.Such composition is spontaneously formed when the components are simply placed into contact with one another.In particular, in case of the isotropic form, the composition is a fluid, clear and stable composition, with particle size smaller than 400 .ANG., containing:(a) from 1 to 50% by weight of the perfluorinated oil;(b) from 1 to 50% by weight of water;(c) from 8 to 60% by weight of at least one surfactant selected from the above defined surfactants;(d) from 6 to 38% by weight of a co-surfactant, selected from (C.sub.3 -C.sub.7)-aliphatic alcohols.In case of the anisotropic form, the composition has a high viscosity, and contains:(a) from 1 to 50% by weight of the perfluorinated oil;(b) from 1 to 80% by weight of water;(c) from 1 to 40% by weight of at least one surfactant selected from the above defined surfactants;(d) from 0 to 19% by weight of a co-surfactant, selected from (C.sub.3 -C.sub.7)-aliphatic alcohols.Furthermore, in case of the isotropic composition this latter finds use also in the biomedical sector.