The control of xylitol production from xylose-grown Debarlyomyces hansenii yeast, which is a very complex biological process, usually requires accurate and demanding analytical HPLC measurements. For this reason, estimating relationships of the main variables of the fermentation process-concentration of xylitol, biomass and xylose-are suggested and studied in this article. The volume of added base for maintaining pH at the prescribed level has been shown to be important for approximate assessment of the biomass concentration and, therefore, for all estimation relationships. Furthermore, replacement of expensive off-line HPLC analyses of xylose by an on-line determined respiratory quotient RQ for control purposes is discussed. On the basis of this, physiological control of xylitol production which takes advantage of on-line classification of the different metabolic states of the culture from easily and cheaply measured variables, has been developed. Data and knowledge obtained from several experiments were evaluated for this reason and results are presented. (C) 2002 Elsevier Science Ltd. All rights reserved.