2-Methylbutanol (2-MbOH) is well known as a genuine flavour compound of apple fruits, whilst 3-methylbutanol (3-MbOH) is not detectable in fresh apples. 3-MbOH is a derivative of leucine metabolism, caused by microbial activities (yeasts, moulds) and, therefore, a characteristic compound of processed apples, such as apple juice, cider or apple distillates. This investigation reports on the GC/MS analysis of 2- and 3-methylbutanol and ethanol from apple juices during spontaneous fermentation reactions. As to be seen from model experiments, there are definite correlations between ethanol and the so called fusel alcohols. Against this background the amount of 3-MbOH in conjunction with the relative percentage of 3-MbOH - in relation to the sum of 2-MbOH and 3-MbOH - are discussed as parameters of apple juice quality. The maximum permitted level of ethanol (3.0 g/l) in fruit juices is commonly accepted as a regulatory principle of apple juice. Taking into account the correlations of EtOH and 3-MbOH, achieved in this investigation, would lead to a tolerable limit up to 4 mg/l of 3-MbOH. Nowadays, however, according to good manufacturing practice apple juices are produced with EtOH and higher alcohol contents far beyond these levels. Ring tests with apple juices - produced under GMP-conditions and focused to the investigated correlations of ethanol with fermentation by products - are recommended. These data should be evaluated in order to define their usability as parameters in quality assessment of fresh apple juice and apple juice from concentrates respectively.