Thermodynamics behaviour of extremes of the chemical potential of substance condensed out of vapor in drop formed on soluble surfactant nucleus at a variable value of surfactant adsorption on the drop surface has been shown. The threshold value of vapor chemical potential determined by maximum of the condensate chemical potential or by the largest of maxima in the case of two coexisting maxima, was found. Results of the analytical theory were illustrated by direct calculations of the condensate chemical potential as a function of the number of condensate molecules in a drop.