Aldehydes are soft electrophiles which are able to interact with proteins, enzymes and DNA through various mechanisms. They are able to stimulate a range of environmental toxicity and adverse health effects. Chemical category formation and the use of read-across to fill data gaps are seen as crucial methods for risk assessment of industrial chemicals under the REACH legislation. Such methods are especially important if the goal of reducing the number of experimental animals used in toxicological testing is going to be met. One of the crucial steps in the development of a chemical category is the definition of the applicability domain of the category in terms of the types of chemicals that should be included in the category. It has been suggested that the use of reactive mechanistic domains can aid in this process for several endpoints including mutagenicity and acute aquatic toxicity. The aim of this study was to classify chemicals into several reactive categories for mutagenicity and acute aquatic toxicity. In addition, mechanistic categories for both endpoints have been compared. A discussion of the requirement for the mechanistic differences between the two endpoints has also been done.