The morphology of long bubbles in horizontal slug flows is experimentally investigated through images obtained with a high-speed camera. The flow conditions resulted in very long and interface-perturbed bubbles, so that a special procedure needed to be developed to correctly identify all of their geometric features. The procedure furnishes a complete characterization of the three-dimensional aspects of the nose, main body, hydraulic jump and tail of bubbles, including their fractal dimension and gas-liquid interface area. Image treatment and numerical computations were carried out with Wolfram's Mathematica 8.0 software system. The results are compared with a previously advanced theory. Characteristic volumes and areas of long bubbles in slug flows are correlated through simple power-law expressions. (C) 2014 Elsevier Ltd. All rights reserved.