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This paper investigates the conditions for the applicability of the geometrical similarity law to impulse breakdown in gases, using statistical methods necessitated by the stochastic nature of the impulse breakdown voltage. Theoretical analysis in the paper is accompanied by experimental results obtained for geometrically similar systems, i.e. for systems with equal shapes of the macroscopic and microscopic electrical field. The experiments were conducted in controlled laboratory conditions, for a wide range of gas pressure and inter-electrode gap values. Two-electrode systems were used, with both homogeneous and non-homogeneous electrical fields, utilizing SF(6), N(2) and He gases as insulators. Standard lightning and switching voltage impulses were applied, as well as ramp-shaped impulses with different slopes. On the basis of the statistical processing of the obtained experimental results, conclusions regarding the conditions for the applicability of the geometrical similarity law to impulse breakdown in gases are drawn.