According to the data of simultaneous gas-phase electron diffraction (GED) and mass spectrometric (MS) experiments the vapour over beryllium dichloride at 547 K consists of monomeric and dimeric molecules in the ratio 97.5:2.5. Molecular parameters were refined for both molecules from GED data. The equilibrium structure of BeCl 2 is linear with the bond lengthr a n h e (Be-Cl) = 1.791(5) A. The thermal-average bond length r g (Be-Cl) is 1.798(4) 9. Relations between different kinds of the refined parameters (r a ,r α , r h e , r c h e , r a n h e ) are discussed. Force constants and vibrational frequencies were obtained for this molecule from GED data alone. AssumingD 2 h symmetry the following parameters were obtained for Be 2 Cl 4 : r g (Be-Cl t ) = 1.828(14) A, r g (Be-Cl b ) = 1.968(20) A, α (Cl t -Be-Cl b ) = 134(4)°. The force constants and frequencies for the dimer were estimated.