CdS nanoparticles were synthesized by the ultrasonic-assisted sol-gel method and characterized by X-ray diffraction (XRD). The Ag-doped CdS (Ag/CdS) nanoparticles were also synthesized by the ultrasonic-assisted sol-gel method and characterized by XRD, X-ray fluorescence spectrometry, scanning electron microscopy, and energy-dispersive X-ray analysis. The calculated sizes and experimental results are in good agreement. The calculated band-gaps of CdS and Ag/CdS were found to be 2.62 and 2.46 eV, respectively. Photocatalytic degradation of Direct Red 264 azo dye was investigated with CdS and Ag/CdS under UV-C and visible light irradiation. Experimental results indicate that the degradation rate of DR 264 follows pseudo second-order kinetics. Also, the intraparticle diffusion coefficients of CdS and Ag/CdS were determined.