Baker's yeast is one of the micro-organisms that is studied most in literature. Therefore, a lot of knowledge on the biochemical pathways and corresponding yield coefficients is available. This knowledge is combined with measurements of oxygen and carbon dioxide in the exit-gas to determine the coefficients appearing in the stoichiometric equations. In this manner, two measurements are sufficient to yield on-line estimates for biomass, glucose, ethanol and the specific growth rate, and information about the (ill-defined) nitrogen source NHq. This is not possible if the yield coefficients are not included in the estimation procedure. A sensitivity analysis illustrates that this estimation scheme is rather insensitive to uncertainties on the yield coefficients.