Protomicelle is a new term in colloid science that refers to micelle-like surfactant aggregates in solutions. Unlike conventional micelles, protomicelles have no critical micelle concentration (CMC) and are formed gradually via adsorption of surfactant molecules or ions on solubilization cores at concentrations significantly lower than the CMC. A completely formed protomicelle looks like an ordinary solubilisate-containing micelle. In this work, the process of monomerization of a given amount of crown-substituted magnesium phthalocyaninate (I) in a premicellar aqueous solution of tetradecyltriphenylphosphonium bromide (II) has been studied by spectrophotometry. It has been shown that compound I monomers are absent in a solution in pure water, while the content of dimers increases as surfactant II is added to the solution. This phenomenon strictly proves the existence of higher-order molecular aggregates of I in aqueous solutions. A new physicochemical parameter of phthalocyanine monomerization has been defined, namely, the monomerization concentration, i.e., compound II concentration that corresponds to the beginning of this process. It has been shown that phthalocyanine monomerization, which has begun long before the CMC of II, continues above the CMC, although at a lower rate. At this stage, monomerization of I proceeds exclusively at the expense of dimers.