Two coupled modes of vibration are sometimes observed with piezoelectric‐ceramic resonators in which a single mode is expected. The coupling is inferred through the presence in the admittance characteristic of two close resonance peaks; it is also seen through a nonexponential decay of resonant vibrations. The effect is attributed to a perturbation of symmetry (such as a flaw in the material or imperfect geometry) that permits coupling of the principal piezoelectric mode to some other secondary mode whose resonance is close in frequency. To determine the requirements for obtaining coupling, a split‐electrode system was used on a thin, rectangular, ceramic plate to measure the resonant frequencies of the first three extensional modes (length, edge, radial) and first three face‐flexural modes, the dimensions being varied from a length/width of 6:1 to 1:1. Coupling between the length mode and second flexural mode was investigated in detail, using a plate of length/width 3.5:1, resonant at 45 kc/sec. Perturbations of symmetry were obtained by adding or removing mass at a corner; coupling could be produced by a change in mass of the order of 10−3of that of the whole resonator. The experiments show the modification of the electrical characteristics of the principal piezoelectric resonance as the coupling to a secondary mode of the same frequency is increased and, also, the effect of coupling two modes of different frequency. The results are discussed qualitatively in terms of an equivalent electrical circuit.