CdS and CdSe are compound semiconductors which have bandgaps of 2.52 and 1.75 eV, respectively. The bandgap of mixed crystals such as CdS[inf 1-x]Se[inf x] (0(less than or equal to)x(less than or equal to)1) is proportional to x. Reliable characterizations of them have become important for applications in solar cell technologies. Although several investigations of the single crystals have been carried out, optical absorption measurements of ceramic CdS and mixed crystals have been a difficult problem. The purpose of this report is to show the results of photoacoustic spectra for ceramic CdS and CdS[inf 0.5]Se[inf 0.5] for different sintering temperatures. Ceramic plates were obtained by sintering over the temperature range from 600 to 950 (degrees)C in N[inf 2] gas. The body of the cell was an aluminum cylinder with a small channel in its periphery in which the microphone was inserted. Modulation frequencies for the measurements were 33, 133, and 257 Hz and the measurements were carried out in the wavelength range of 330--1200 nm. The signal intensity and phase spectra shift toward the high-energy regions and the slope of the absorption edge becomes steeper as the sintering temperature is increased. Moreover, the phase spectra show minima whose positions depend on sintering temperature, because of the increase of the recombination rate.