Field Emission Gun Transmission Electron Microscopy (FEG TEM) is used to measure solute atom segregation to stacking faults in a Cu- 7.15 at% Si alloy annealed at temperatures of 275°C, 400°C and 550°C. A highly localised increase in the silicon concentration was detected at the stacking fault plane. Segregation of the higher valence silicon atoms increases the local electron to atom ratio in the crystal thereby lowering the stacking fault energy as expected for a Cu-Si alloy system. Measurements across low angle boundaries showed hardly any change in solute concentration, which suggests that the segregation observed at stacking faults is due to a genuine chemical interaction rather than an elastic interaction which can occur in for example high angle grain boundary segregation. The segregation was greatest for the 275°C annealed alloy, where the enrichment is more than 2 at% Si above matrix composition, and was found to decrease monotonically with increasing temperature. The binding energy, as determined from a McLean isotherm, was measured to be -0.021 eV/atom. This is significantly different to the theoretical estimate of -0.0022 eV/atom, as calculated by data on phase equilibria. This could be due to several reasons including experimental error, assigning thermodynamic data of a bulk phase to a stacking fault and inadequacies in the Suzuki segregation model itself.