Monomeric and dimeric MgCl2 have been investigated by high-temperature electron diffraction and ab initio calculation at the SCF and MP2 levels of theory. The effect of the size of the basis set, and particularly the number of polarization functions and their exponents, has been carefully investigated. The basis set size was increased until a convergence in the monomer bond length was reached at the TZ5P2f(+)/MP2 level. The highest level of calculation for the dimer was of DZP(+)/MP2 quality. Harmonic vibrational frequencies were calculated and normal-coordinate analyses were performed for both monomeric and dimeric molecules. For the electron diffraction determination of the dimer geometry, constraints based on the quantum chemical calculation, such as the differences of the Mg-Cl bond lengths, were incorporated into the analysis, as the dimer was present only as a minor component of the vapor (12.8 +/- 1.3 mol %). Monomeric MgCl2 is linear with a bond length of r(g)(Mg-Cl) = 2.179 +/- 0.005 Angstrom. The converged calculated bond length, r(e)(Mg-Cl) = 2.169 Angstrom, is consistent with the value estimated from electron diffraction applying vibrational corrections, r(e)(M)(Mg-Cl) = 2.163 +/- 0.011 Angstrom. This agreement, however, can only be obtained with large basis sets; standard bases of DZP quality and standard exponents give much larger bond lengths than the experimental value. The calculated equilibrium structure of the dimer has a four-membered planar ring with two bridging chlorines (D-2h symmetry). At the high temperatures used, the dimer appears strongly puckered if a standard electron diffraction analysis is used, but the best fit to the experimental data was obtained by adopting a dynamic model, in which the calculated rather anharmonic ring-puckering potential, together with the associated changes in the other geometrical parameters, was used as an additional constraint. Only a very minor increase in bond length with bending was found for the monomer, whose bending potential is close to harmonic.