Cadmium dioleylphosphate (CDOLP) and sodium dioleylphosphate (SDOLP) are expected to form rigid microemulsion droplets, They were used as surfactants in place of sodium bis (2-ethylhexyl) sulfosuccinate (AOT), which has been often used for the preparation of cadmium sulfide (CdS) ultrafine particles in microemulsion (ME), CdS ultrafine particles were prepared by mixing hydrogen sulfide solution with the ME composed of CDOLP and SDOLP, The growth process of the particles through their coagulation is restricted more strongly, and the size distribution of the obtained particles was sharper than those in ACT-ME, Furthermore, in CDOLP/SDOLP-ME, the average number of cadmium ions per microemulsion droplet is larger and the number of contaminating "empty" RIE droplets which contain no particle is much smaller than in AOT-ME systems. Dioleylphosphates are proper surfactants for the preparation of ultrafine particles with sharp size-distribution, because particle coagulation is restricted in the system. The restriction of the coagulation between larger particles is attributed to two mechanisms: 1) It is hard for larger particles to be exchanged between two fusing ME droplets through the "open channel" between them, 2) The ME droplets with the larger particles can hardly coalesce with the other ME droplets.