The structure and conformational composition of 1,3-dichlorobutane and 1,3-dibromobutane have been studied by gas-phase electron diffraction (ED) at 28 and 23 degrees C, respectively. Results from ab initio molecular orbital calculations (HF/6-31G* and HF/6-31G*/Binning, respectively) have been used as constraints in the ED analysis. These molecules may in principle exist as a mixture of nine different conformers, five of these were included in the theoretical model for the electron diffraction investigation. For both compounds the investigation indicated that the G+G+ conformer dominated the conformational composition in the gasphase (54 +/- 20% and 60 +/- 22%, for 1,3-dichlorobutane and 1,3-dibromobutane, respectively). The distributions among the other conformers are more uncertain, because some of them give nearly equal distance distributions.