AbstractOver the past two decades there has been increasing interest in the use of the concept of a quality factor Q, introduced by Drouzy and co-workers, as a single parameter for assessing the‘quality’of Al alloys. This quality factor is defined as Q=σUTS+150 log E, whereσUTSis the ultimate tensile strength and E is the elongation, and has so far been used mainly for the description of the Al–7Si–Mg alloys A356 and A357. The current study was carried out to assess whether the concept could be extended to the higher strength Al–4·5Cu casting alloys A201 and A206. It was concluded that the quality factor does not apply to A201 and A206 in an analogous manner, and thus the concept is not transferable at the present time. It is recommended that the use of the quality factor for A201 and A206 be discontinued until it has been more fully investigated. In the meantime, an alternative definition, Q=σ0·2+kE, whereσ0·2is the 0·2% proof strength, as proposed in the present paper, seems useful and appropriate. This new definition may have wider applicability, but requires further investigation. The use ofσ0·2versus E maps is recommended to encompass the whole range of properties of interest to the designer. The area defined byσ0·2×E, and clearly shown on theσ0·2versus E map, is proportional to the energy required for ductile fracture. This is revealed to be over an order of magnitude greater in A201 than in A356 and A357 alloys.MST/3279