In previous work, the normal surface displacement patterns of thick barium titanate disks vibrating in various normal modes have been obtained using multiple‐beam optical interference. This technique has now been used to determine the effect of loading a disk with a liquid in contact with one plane surface. It is found that the discrete modes observed with an unloaded disk begin to merge so that the vibration patterns show a continuous transition with changing frequency as one passes from one mode to the next in the “thickness resonance” region. The radiation damping of the individual modes is appreciably less than the theoretical value, but the bandwidth covered by the group is appreciably greater than is to be expected from the simple theory of thickness resonance. In the case of a quartz disk, two modes with a frequency separation of 0.5% merge fairly completely when the disk is loaded, and the vibration patterns indicate composite motion of a more uniform character than that of either mode alone.