The limited ductility of wrought Mg alloys persists to be a problem, both in its fundamental understanding and for the application of these alloys. Therefore, the present study focuses on determining the operative deformation mechanism in five different Mg alloys namely, pure Mg, Mg-0.1Ca, Mg-1Al-0.1Ca, Mg-1Al-0.05Ca and Mg-2Al-1Zn-0.1Ca (AZX2101) alloy (all in wt.%). The investigated alloys, selectively heat-treated, exhibit characteristically similar textures and grain sizes; this allows a direct comparison between observed deformation mechanisms. The deformation mechanisms were investigated using room temperature stress relaxation test (SRT) (tensile). SRTs were carried out at different strain levels ranging from 1 % to 15 % depending on the achievable ductility in the selected materials. Analyses of the obtained data yield the values of thermal stress ($\sigma_{th}$) and apparent activation volume (V) for all investigated materials. These values depict the thermal activation phenomena during deformation; $\sigma_{th}$ attributes mainly to short-range order, forest dislocation, impurity, and solute atoms interactions. The activation volume expresses the volume, physically swept by a dislocation from a ground equilibrium state to an activated state after the deformation. A higher value of V denotes higher-order deformation mechanism e.g. cross-slip, climb. The magnitude of V and $\sigma_{th}$ are, respectively, 5 times higher and 7 times lowers in AZX2101 compared to pure Mg. These variations of $\sigma_{th}$ and V vs strain in the investigated materials indicate that addition of Ca and Zn increases the chance for kink-pair formation and thus the cross slip probability. In contrast, addition of Al increases the athermal stress and hardening in the Mg alloys. These observations are further strengthened using Haasen plot calculations, based on the variation of stress ($\sigma$) and V. The slope and intercept values obtained from this plot describes strain rate sensitivity and extent of thermal activities, respectively.