The effect of stacking fault energy (SFE) on the evolution of microstructures during room temperature tensile testing has been investigated at two strain rates of 8.3 x 10(-4) and 1.7 x 10(-1)/s in pure copper, Cu-2.2%Al, and Cu-4.5%Al alloys with SFE values of, approximately, 78, 20 and 4 mJ/m(2), respectively. The mechanism of deformation changes from simple slip leading to cell formation in the high SFE metal, Cu, to overlapping and/or intersecting deformation twins in low SFE alloy, Cu-4.5%Al. The effect of strain rate is such that it results in rather poorly defined cell boundaries in copper, with a smaller cell size at higher strain rates for similar grain sizes and strain values. The alloys deform by twinning and the propensity of deformation twins increases with both a decrease in SFE value and increase in strain rates.