A general coupled amplitude equation is developed for surface acoustic waves (SAW) from coupled‐mode theory. The derivation accounts for the surface propagating character and bulk decay pattern of the uncoupled modes. The general equation is applied to interactions arising from the weak nonlinearities of the materials supporting SAW’s. Coupling contants K of the nonlinear interactions are shown to be determined from the nonlinear material constants and normalized particle velocity, electric, and strain fields of the linear normal modes. For conservative coupling, a single coupling constant describes a given three‐wave interaction. For a given mode &agr;, the coupling constant to two other modes (&bgr;,&ggr;) scales as the product of the frequencies of modes &bgr; and &ggr; (&ohgr;&bgr;&ohgr;&ggr;). Theory shows how one‐dimensional single‐nonlinear‐parameter models can approximately account for multiharmonic generation onYZ‐LiNbO3. Applying the theory to experiments on second‐harmonic generation, we find the nonlinear coupling constant forYZ‐LiNbO3is ‖K‖≊1.1×109m/sec2 W1/2at 50 MHz. Similarly for (11¯0)‐(001) Bi12GeO20, we find ‖K‖≊1.4×108m/sec2 W1/2at 50 MHz and ‖K‖≊5.3×108m/sec2 W1/2at 100 MHz. The nonlinear coupling constant units are for interacting field amplitudes normalized to the square root of action density integrated over depth (W sec2/m)1/2.