The behaviour of Pichia stipitis, Pachysolen tannophilus, Candida sheh
atae and Candida parapsilosis was investigated to select the most suit
able yeast to convert xylose either to ethanol or to xylitol, with lit
tle or no formation of by-products. The aeration rate was used as a va
riable parameter. P. stipitis and C. parapsilosis were the most effect
ive producers of ethanol and xylitol, respectively, both reaching prod
uctivities at very low levels of oxygenation. With P. stipitis, better
ethanol productivity was attained under microaerobic conditions (K(L)
a=4.8 h(-1)) while with C. parapsilosis high yields and rates of xylit
ol production were detected at K(L)a values of about 16.3 h(-1). P. ta
nnophilus and C. shehatae showed lower performances under all conditio
ns used while changes in oxygenation modified the ratio of ethanol to
xylitol produced by these yeasts, suggesting that they are more depend
ent on the oxygen power input than P. stipitis and C. parapsilosis. Th
e influence of oxygen transfer rates on ethanol and xylitol formation
with the best producers is discussed.