It has recently been shown that a consideration of the stability of a stratified flow and of the stability of a slug has provided accurate predictions of the critical liquid height of a stratified flow for the transition to a slug or plug flow in horizontal and downwardly inclined pipes. Two diameters and viscosities of 1-70 cp were considered. Predictions of the critical superficial liquid velocity are not so accurate. This paper partially examines this issue by presenting measurements of the mean pressure gradient and of the time-varying holdup for air and water flowing in a horizontal 2.54 cm pipe. Important contributions are the use of viscous long wavelength theory to predict the initiations of roll waves and the discovery of a range of liquid flows (in the pseudo-slug regime) at which the liquid holdup of the stratified layer is close to the critical and is not changing strongly with liquid flow. A partial explanation for this behavior is given which recognizes that the liquid layer over which roll waves are propagating is below that required for the stability of a slug. Slugs will appear if conditions are favorable for the coalescence of roll waves. (C) 2002 Elsevier Science Ltd. All rights reserved.