The authors consider steady-stat e frequency spectra for Alfvén and magnetohydrodynamic waves created when the waves interact non-linearly. The principal form of non-linear interaction is scattering by plasma particles. A study is made of the stability conditions for the power-type spectra WωΩ = (1/ων)WΩ. It is demonstrated that power-type spectra with ν ≥ 1 are stable; at ν = 1;2 the power-type nature of the spectrum is not disturbed during establishment of the steady state, as distinct from other values of ν.