The effect of frequency modulation during stochastic ion heating induced by lower‐hybrid waves is examined. The modulation occurs either in the ion‐cyclotron frequency because of the variation of the magnetic field in toroidal devices, or it can be externally imposed on the frequency of the lower‐hybrid waves. It has already been observed numerically [Phys. Fluids27, 184 (1984)] that a small variation in the ion‐cyclotron frequency can induce velocity diffusion for wave amplitudes well below the stochasticity threshold in a uniform magnetic field. Here a detailed study reveals that to the lowest order in the small parameters, the modulational effects can be incorporated in a two‐dimensional Hamiltonian. This allows the derivation of the new stochasticity thresholds. It is found that a small amount of modulation, &Dgr;&ohgr;/&ohgr;≲1%, produces an order of magnitude reduction in the stochasticity threshold relative to the constant frequency case. The stochastic regime in velocity space also grows in size, resulting in a considerable increase of number of heated particles in the case of devices with modest aspect ratio. Both ion‐cyclotron and wave‐frequency modulation lead to similar results. The modulation of the wave frequency offers the ability to control and optimize the modulation parameters and is proposed as a method to enhance radio frequency (rf) heating.