It is shown to be possible to identify dominant modes of radiationless relaxation of ions in crystals by comparing photoacoustic spectra with fluorescence excitation spectra. The dominant decay mode for Cr3* in MgO is shown to be quite different from that found in ruby. Photoacoustic spectroscopy (PAS) has recently been shown to be a useful technique for investigating radiationless relaxation processes of ions in crystals [l-3] _ We report here the results of PAS experiments on trivalent chromium ions in four different host crystals; A1203, MgO, SrTiO,, and BaTiO,. By comparing the relative intensities of the two broad absorption bands of Cr3+ in the PAS spectrum with their intensities in the fluorescence excitation spectrum, the dominant mode of relaxation back to the groud state can be determined. This is shown to be quite different for chromium doped Al203 and MgO while in SrTiO, and probably BaTiO, the signals are masked by host absorption. The excitation light for these experiments was provided by a 1000-W tungsten/halogen "sun gun" after being passed through a l/&neter monochromator with a resolution of 200 A and chopped at 100 Hz. For obtaining excitation spectra, the fluorescence emission in the region of the R lines of Cr3+ was monitored using a second I/4-meter monochromator and a cooled RCA C3 1034 photomultiplier tube. The spectra were recorded after being processed by a lockin amplifier. To obtain the photoacoustic spectra the samples were placed in an acoustical cell containing a l-in. electret microphone. The signal was again pro-* Sponsored by the U.S. Army Research Office.