In the first part of the paper it is shown how, on the basis of a different form of the energy equation, a more compact derivation of the Rott theory of thermoacoustic effects [N. Rott, Z. Angew. Math. Phys. 20, 230–243 (1969)] can be given. In the second part, a weakly nonlinear theory is built starting from the same formulation and the steady-state amplitude of the oscillations setting in above the stability threshold is calculated. [Work supported by the Physical Acoustics Program of ONR.]