The stress-induced linear dichroism in the absorption of the F center in KC1 and the U center in KBr has been measured in the range from 7 to 120°K. The moments analysis of Henry, Schnatterly, and Slichter has been used to interpret the results. For the F center, a measurement of the changes in the third moment of the F band allows the determination of the separate contributions to the second moment by lattice vibrations of Ti + , r 3 + , and T 5 + symmetry. Within the harmonic approximation, the temperature dependence of these separate contributions defines an average vibrational frequency for each symmetry mode broadening the band. The measured frequencies, an = 2.3±0.lXl0 13 sec" 1 , a> 5 = 1.95±0.1X10 13 sec" 1 , and «,= 1.4±0.1 X10 18 sec" 1 , lie in the acoustic and low-frequency optical region of the host-lattice vibrational spectrum. The stress response for the U center was found to be qualitatively identical to that of the F center. The changes in the first moment of the U band determine the excited-state coupling to unit strains of r 3 + and T& + symmetry. From the changes in the third moment, it was found that cubic vibrations dominate the broadening of the U band. A broad peak in the dichroism pattern to the high-energy side of the U band is interpreted as evidence for transitions to higher excited states of the U center analogous to the iT-band transitions associated with the F center.