Computer aided aroma design (CAAD) is likely to become a hot issue as the REACH EC document targets many aroma compounds to require substitution. The two crucial steps in computer aided molecular design (CAMD) are the generation of candidate molecules and the estimation of properties, which can be difficult when complex molecular structures like odours are sought and when their odour quality are definitely subjective whereas their odour intensity are partly subjective as stated in Rossitier's review [K.J. Rossitier, Structure-odour relationships, Chem. Rev. 96 (1996) 3201-3240]. In part I, provided that classification rules like those presented in part II exist to assess the odour quality, the CAAD methodology presented proceeds with a multi-level approach matched by a versatile and novel molecular framework. It can distinguish the infinitesimal chemical structure differences, like in isomers, that are responsible for different odour quality and intensity. Besides, its chemical graph concepts are well suited for genetic algorithm sampling techniques used for an efficient screening of large molecules such as aroma. Finally, an input/output eXtensible Markup Language (XML) format based on the aggregation of Chemical Markup Language (CML) and ThermoML enables to store the molecular classes but also any subjective or objective property values computed during the CAAD process. (C) 2008 Elsevier B.V. All rights reserved.