The effect of a volumetric nanosecond discharge on detonation cell size was demonstrated experimentally in a detonation tube test rig. The experiments were performed in CH4:O-2:Ar=1:2:2 and CH4:O-2:Ar:H-2=3:7:8:2 mixtures, at 180 mbar and 120 mbar respectively and ambient temperature. The plasma was generated by two consecutive pulses of -50 and -32 kV amplitude on the high-voltage electrode and 25 ns pulse duration. The analysis of the detonation cell size with and without plasma generation was performed via sooted-plate technique. The detonation cell size was reduced by a factor of 1.5-3, while passing through the region of the discharge. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.