Determination of deterioration of bottled beer in trade packages may be a rather complicated task, so an investigation was made of formation of furfural as an index of change. Color increase, sometimes advocated as a measurement of oxidation or browning, was followed concurrently. A fluorometric method for the determination of furfural was used, based on the fact that furfural reacts with o-aminothiophenol to form 2-(2-furyl) benzothiazole which shows an intense fluorescence in strong acid solution. The furfural was collected by steam distillation in a micro-Kjeldahl, which separated the furfural from those other carbonyl compounds which would otherwise interfere. The furfural content of fresh, unpasteurized beers tested was below 15μg/l., but the level increased significantly on pasteurization and dramatically on exposure to warm ambient temperatures. Color was affected far less. Increases in furfural in beer held at 25°C followed a first-order reaction rate pattern, but those in beer at 40°C were greatly accelerated and appeared to be by different mechanisms or reactions. There is a correlation between major changes in furfural content and decreases in taste panel scores, though the furfural levels were never above the taste threshold for this compound.