Justification of the hypothesis about the avalanchemechanismof vacuumbreakdown is examined. For that purpose, values of dc and impulse breakdown voltage have beenmeasured under well defined laboratory conditions, in the pressure range from 10-6 mbar to 1 bar, and for inter-electrode distances from0.1 mm to 1 mm. Variable parameters of the experiment included the type of the residual gas and the material of the electrodes in the system used for forming a homogeneous electric field. Statistical analysis of experimental results and their comparison to the predictions of a theoretical model, attest that the avalanche mechanism of breakdown does come about in low vacuums.