Abstract A method based on the interfacial polycondensation of isocyanates has been used to prepare microcapsules containing an aqueous solution of benzalkonium chloride. Since the compound to be encapsulated is amphiphilic, particular attention was paid to the formulation of a starting system for encapsulation. To this end, we investigated the influence of the hydrophilic-lipophilic balance (HLB) of both the surfactants and the oil on the initial emulsion. Our results demonstrate that designing of the formulation in accordanc with the HLB concept of Griffin ( J. Soc. Cosm. Chem., 1 (1949) 311–326) constitutes a good approach to the choice of the components of the microencapsulation system. The phase diagram constructed for a pseudo-ternary system showed that the area of the domain of the lipophilic continuous phase emulsion decreased with increasing concentration of benzalkonium chloride. This observation can be explained as being due both to the amphiphilic nature of the benzalkonium chloride and to modification of the aqueous solubility of the surfactants in its presence. The encapsulation yield of benzalkonium chloride was high (60%). The remaining 40% may either be present in the lipophilic phase or become incorporated into the shell of the microcapsules.