The authors analyse the self-modulation of large amplitude Langmuir waves due to weak relativistic corrections to the plasma frequency and also to the nonlinear coupling of the wave with ion quasi-modes. It is shown that the wave is unstable to long wavelength perturbations with the instability occurring in the kinetic regime where (me/mi)(Ti/Te) equivalent to k2 lambda 2DE which is different to the quasi-static or hydrodynamic regimes.