We characterised different flow regimes and their transition through an objective and quantitative assessment of flow behaviour by applying four signal process techniques (auto-correlation, power spectral density (PSD), Shannon entropy, and permutation entropy) onto pressure signals of counter-current two-phase flows in a 170 mm x 70 mm concentric vertical annulus. Differential pressure values were measured using a 100 Hz sampling rate with 30 second sample sizes in 147 experiments at superficial gas velocities within 0.043-5.387 m/s and superficial liquid velocities within 0.016-0.093 m/s. Video images were also captured during all flow experiments. Using the signal analysis techniques, we could overcome the difficulties and uncertainties that accompany subjective visual observations in the characterisation and classification of flow regimes. Also, the signal analyses of the pressure signals allowed us to provide insights into the mechanisms responsible for formation of each flow regimes and their transitions. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.