Despite extensive studies on the aging behaviour of Al3Sc containing alloys, the underlying mechanism of the precipitation strengthening is still not well understood. In particular, the transition radius at which particles become non-shearable is not known. In this work, the work hardening behaviour of an Al-2.8Mg-0.16Sc (wt%) alloy has been characterized for different stages of aging and the corresponding slip line features at the surface of strained specimens have been examined using Nomarski interference contrast. Moreover, the work hardening behaviour is discussed in the framework proposed by Kocks, Mecking and Estrin. It is proposed that changes in macroscopic work hardening behaviour can be used as a signature of the shearable/non-shearable transition.