Abstract: The paper presents the results of studying the characteristics of a slow ionization wave (IW) arising at the initial stage of breakdown in a long discharge tube under reduced pressure. The discharge tube is a Philips TUV-30W mercury lamp with an electrode spacing of 80 cm and an inner diameter of 23 mm. The tube is filled with argon at a pressure of 2–4 Torr (nominal data) and mercury vapor. One of the electrodes is grounded, and, to the second one, positive or negative voltage pulses with an amplitude of 2 kV, a leading edge duration of ≈0.5 μs, and a repetition frequency of 1 Hz are applied. The IW velocity and the time dependence of the intensities of the Ar, Ar⁺, and Hg lines in the IW at different distances from the high-voltage electrode are measured. It is shown that the velocity of a positive IW (3 × 10⁷–5 × 10⁷ cm/s) is higher than the velocity of a negative IW (1 × 10⁷–1.8 × 10⁷ cm/s). The magnitude of the electric field in the front of an IW is evaluated from the comparison of the measured and calculated intensity ratios of the Ar, Ar⁺, and Hg lines. It is shown that, in a positive IW, the magnitude of the reduced electric field (260–450 Td) is noticeably larger than in a negative one (120–165 Td).