Protomicelles represent a new concept of colloid science relevant to micelle-like aggregates of surfactants in solutions. In contrast to ordinary micelles, their formation requires no critical micelle concentration (CMC) and proceeds gradually through the adsorption of surfactant molecules or ions on a solubilization core at concentrations much lower than the CMC. A completely formed protomicelle looks like an ordinary solubilisate-containing micelle. Protomicelles can be formed on phthalocyanine monomers and dimers. In this work, the role of protomicelles in monomerization of crown-substituted magnesium phthalocyaninate (I) in aqueous sodium dodecyl sulfate (II) solutions has been studied spectrophotometrically at compound I concentrations of 0.38 and 1.2 μМ. Empirical methods have been developed for determining a new parameter, monomerization concentration (concentration of II required to initiate the monomerization of I), and it has been shown that the monomerization concentration decreases with an increase in the concentration of I. Another new parameter defined as the concentration of II corresponding to nearly complete monomerization of I has been studied in comparison with the CMC of II. For this purpose, the CMC has, for the first time, being measured (by conductometry) in the presence of protomicelles. The latter circumstance has been shown to decrease the CMC. At the same time, it has been revealed that the concentration corresponding to the completion of monomerization in the studied system nearly coincides with the CMC.