CdS quantum dots (QDs) were synthesized by the ultrasound-assisted chemical precipitation technique. The structure analysis revealed the presence of bi-structural cubic and hexagonal phases with an average crystallite size of 3 nm. The N₂-adsorption isotherm exhibited the evolution of meso-/macro-porous interfaces with a pore size of 7.56 nm and a surface area of 44.41 m²·g^-1. The improvement of the quantum size effect in CdS QDs resulted in the increase of optical bandgap to 2.52 eV compared with the corresponding bulk phase. However, the analysis of long-tail states absorption revealed a very small Urbach energy of about 76 meV compared with CdS QDs prepared by other techniques. The as-synthesized CdS QDs revealed high room-temperature DC conductivity of 2.56×10^-6 Ω^-1.m^-1 and very small activation energy of 268 meV facilitating tunnelling of the thermionically excited carrier through the high bandgap of CdS QDs. The frequency-dependent behavior of AC conductivity and dielectric constant (εr) of CdS QDs were investigated at different temperatures in the range from 303 K to 453 K. It was observed that both AC conductivity  and εr were improved with increasing temperature up to 363 K followed by a sudden decrease at higher temperatures.