Equivalence ratios measured with a laser induced plasma spectroscopy (LIPS, also referred as LIBS) are reported in two different setups. First, a small premixed turbulent burner is used to address fundamental issues concerning the LIPS technique. It is shown that hydrogen excitation within the created plasma is the key parameter to measure in order to retrieve correctly equivalence ratio measurements. Results compared with a spark energy classification strategy show better results with excitation classification, as variations in ratio between the different lines come not only from gaseous concentration but also from plasma's characteristics. Using spectra from 450 to 800 nm allows the determination of two independent emission ratios to improve single shot accuracy. The developed approach is afterwards applied to phase-locked measurements of equivalence ratio in a lean premixed combustor, for which strong thermo-acoustics oscillations exist. This combustor runs with methane–air, preheated at 700 K and with a typical equivalence ratio of 0.50, for which the sound pressure levels of the oscillations are 170 dB. Measurements at the inlet of the combustor reveal strong correlations between fluctuations of the incoming stoichiometry and pressure fluctuations. It is shown that stoichiometry changes within one oscillating cycle of about 3%. Those changes are crucial for the flame dynamics as dealing with very lean mixtures.