Instabilities leading to the breakup of low velocity liquid jets are investigated experimentally. Pulsed shadowgraphy was used to measure local quantities on the wavy shape of the jets with an anamorphic imaging setup. Statistical recording is employed to determine the mean characteristics of the instabilities and time-resolved recording is used to follow individual waves along the jet axis. Particular attention has been paid to the changes that occur in the breakup process at the critical point of the stability curves for jets under internal flow breakup mode regime. For such jets, the critical point is not driven by aerodynamic effects and is not correctly predicted by available theories. It is shown that these jets follow the linear theory of Weber up to the critical point. The growth rate and the wave number increase steeply after the critical point. Evidence is put on the role of the relaxation of the velocity profile in the stability of the jets under internal flow breakup mode regime. (C) 2003 Elsevier Science Ltd. All rights reserved.