The microcasting process is a scaled-down investment casting process in which molten metal is pressure-infiltrated and directionally solidified in water-soluble moulds. It was previously developed to produce metallic microwires with a diameter (D) between 7 and 120 µm, characterized by a high surface quality and aspect ratio. In tension, the yield stress of aluminium (99.99%) microwires (initial dislocation density ~8*10^11 m-2) scales with the inverse of the microwire diameter and becomes highly stochastic at the smallest diameters. The plastic deformation behaviour is intermittent, suggesting that deformation progresses through the repeated activation and blockage of single-arm sources. The objective of this work is to contribute to the general understanding of plasticity size effects, using monocrystalline microcast wires. The focus is placed here on the contribution of thermal activation and initial dislocation content on the overall plastic deformation behaviour. In this frame aluminium (99.99% and 99.999%) and Al-2wt%Mg microwires with a diameter between 12-125 µm were produced and tested. Thermal activation was studied through (60 s) stress relaxations applied at different load levels throughout a tensile test. Single slip microwires (D<120 µm) deform extensively in Stage I of single crystal deformation, unlike their macroscopic aluminium counterparts. The thermally activated deformation behaviour was thus probed both in Stage I and Stage II. A microwire stress relaxation consists of a continuous logarithmic stress decrease interposed with sudden load drops the statistics of which are similar inside and outside of relaxations. From the continuous part of the stress relaxation an (apparent) activation area and the work provided by the applied stress to aid a dislocation overcome an obstacle under the action of thermal activation can be calculated. The work is found to double as deformation progresses from Stage I to Stage II. Aluminium microwires annealed at 500°C under protective atmosphere have in principle a lower initial dislocation density as well as a visibly thicker oxide layer than as-cast microwires. Annealed aluminium microwires oriented for single slip with D~14 µm show a significant increase in yield stress compared to as-cast microwires of similar size and orientation. Plastic deformation progresses mainly through local events of significant amplitude; sufficient dislocations are seemingly not available at this diameter to trigger long-range interactions, leading to a lower overall strain to failure than their un-annealed counterparts. Larger annealed microwires (D~22 µm) sustain on the other hand more plastic deformation compared to similar as-cast microwires. The oxide layer along the surface leads to an overall increase of the flow stress in all annealed microwires, the magnitude of which can be measured as the athermal contribution to the flow stress in a Haasen plot of the annealed microwires, and is in the range of 4-6 MPa, thus roughly doubling the flow stress for a selection of the microwires.