Self-excited acoustic oscillations in a laboratory combustion system containing a premixed flat flame are investigated. The flat flame is stabilized by a copper matrix burner with about 700 deep ports and burns in a resonator tube (diameter 30 nm) of the same area. The system may therefore be treated one-dimensionally. The perturbing mechanisms under consideration are the oscillating reactant input and the oscillating heat transport from the flame to the burner matrix due to the particle displacement (which can be derived by integrating the particle velocity of the standing soundwave). No oscillations are found if the velocity of the reactants is equal to or larger than the laminar flame speed. The experiments show an instability range in the plane of equivalence ratio φ and oscillation period T similar to other studies using premixed gas-fired rocket chambers. However, in our system there is a definite limit with a minimum oscillation period of 1.2 ms; no oscillations with smaller periods are found. Acoustic measurement of the burner impedance shows a change of sign (±) of the reactance (imaginary part of impedance) exactly at this limiting oscillation period. By analogy to electric line theory an “inductive” and a “capacitive” burner reactance can be defined. Oscillations were only observed at an inductive burner reactance. Under the acoustical condition the heat (or temperature) oscillation is exactly in phase with the pressure oscillation if there is a negligible combustion time lag. The criterion set up by Rayleigh is fulfilled.