Nanosized CdS spheres with diameters of about 25, 40, and 50 nm have been synthesized in three inverse microemulsions formed with a dendritic amphiphilic copolymer (H2O/SD31/n‐butanol/n‐heptane) through the reaction of cadmium chloride (CdCl2) with the S2– ions that were slowly released from thioacetamide (TAA). X‐ray powder diffraction (XRD), transmission electron microscopy (TEM), and UV/Vis absorption and photoluminescence (PL) spectroscopy were used to characterize the CdS nanospheres obtained. TEM and XRD studies showed that monodisperse CdS nanospheres with the zinc blende structure were produced, and the size of the CdS spheres increased with increasing water‐core size because of the increasing [water]/[dendrimer] mass ratio (ω). The blue‐shift of the maxima in the UV/Vis absorption and PL spectra indicated that the CdS spheres consisted of primary CdS nanocrystals showing a quantum confinement effect. The viscosities of the three microemulsions were also determined by rheological measurements. The viscosity of the inverse microemulsion system decreased as ω increased. The water core of the inverse microemulsion provided a soft template for the precipitation of CdS spheres.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)