This paper presents a novel approach to study in vivo cellular kinetics by the combined use of magnetic resonance (MR) spectroscopy and Odum's system modeling, which includes the entire process of substrate degradation and product formation. The experiment was conducted directly in the MR-probe, where microorganisms (Klebsiella planticola) were incubated with high (13)C-glucose and (13)C-xylose concentration. Sugar consumptions and (13)C-ethanol production were followed for 17 h and MR-spectra were acquired every 30min without external sampling. Fitting of data allowed evaluation of sugar degradation, ethanol formation, cell activation induced by substrates, as well as cell inhibition induced by ethanol. The dynamics related to the diauxic growth of the bacterium under study were analyzed in terms of different kinetic behaviors. This approach gives not only a good fit of theoretical to experimental results, but is able also to interpret the way substrates, cells and products interact with each other. The proposed methodology can be applied to different living systems to study complex novel mechanisms.