To determine characteristics associated with stability, 35 commercial beers were aged naturally. Chemical instability, as determined by increase in aldehyde concentration during aging, was associated with beer color. In particular, darker beers produced disproportionately high levels of Strecker aldehydes. Other attributes of fresh beer showed little or no correlation with stability. The relationship between color and instability was confirmed experimentally by brewing and fermenting worts of different color, which were otherwise identical. Increases were noted also for the non-Strecker aldehydes furfural and hexanal during aging but concentrations were not influenced by beer color. Potential aldehyde precursors: higher alcohols and amino acids, were not higher in the dark worts and the higher aldehyde concentration is therefore not dependent on pre-cursor concentration. Supplementation of the amino acid isoleucine to fresh beer promoted the formation of 2-methylbutanal particularly in dark beers, presumably due to increased Strecker degradation. Conversely, addition of the higher alcohol 2-methylbutanol led to higher concentrations of 2-methylbutanal irrespective of beer color. Increased Strecker degradation reactions may explain, at least in part, the higher level of Strecker aldheydes observed in dark beers after aging.