The stoichiometry of bioprocesses and the relations between the stoichiometric and experimental yield coefficients are described. General relations resulting from mass and energy balances which describe complex processes of microbial growth and product formation illustrated by a simple metabolic model are also presented. The limitations for theoretical biomass and product yield coefficients are analysed. A case study of respire-fermentative ethanol, production by yeast Saccharomyces cerevisiae for the comparison of theoretical and experimental yield coefficients is presented.