In this paper, experimental results obtained with Laser Induced Plasma Spectroscopy to retrieve local compositions are presented for an ambient pressure up to 5 MPa in a still cell. Well controlled mixtures of gases are introduced and plasma is obtained with the fundamental emission of a pulsed Nd:YAG laser. Simultaneously, plasma shape and spectrally resolved data are taken with a temporal resolution down to 2ns. First, the temporal evolutions of a high-pressure nitrogen plasma are analyzed as function of spark energy. It is shown that plasma changes orientation from an elongated shape parallel to the laser line to a perpendicular one in a very short time. Results are reported for both spatial and spectral variations. Afterwards, the effects of increased carbon concentration are discussed in both shape and spectra. It is seen that strong intensity due to the molecular CN emissions appear for the high-pressure case. From those experiments, calibration strategies are proposed to get equivalence ratio under high pressure conditions with a ratio of CN versus Nitrogen. The delay between plasma and measurements is set to 2000ns and the signal is integrated for 5000ns, so as to yield a good signal to noise ratio and a good sensitivity of the technique to changes in mixture fraction. Calibration curves are reported for equivalence ratio up to 1.25 and for pressure from 0.5 to 5.0 MPa.