An electron beam densitometer has been used to investigate the behavior of a conventional 1⅛‐in. i.d. shock tube operating at initial pressures of the order of 1 mm Hg. These experiments show that such a shock tube does not perform as predicted by simple theory. Most of the experiments were performed in argon with shock Mach numbers ranging between 1.2 and 7.0. The most striking observation was that for a given shock velocity,Ms= 1.6, the distance between the shock wave and contact surface as observed at the densitometer was proportional to initial pressure and independent of expansion chamber length over a tenfold range of tube length. At an initial pressure of 0.5 mm Hg the time interval between the arrival of the shock and the contact surface varied between 600 &mgr;sec atMs= 1.2 and 20 &mgr;sec atMs= 7.0. The diaphragm pressure ratio (Ar ‐ Ar) required to produce a shock of velocityMs= 1.6 varied from 200 at an initial pressure of 0.25 mm Hg to 20 at an initial pressure of 50 mm Hg. For a given diaphragm pressure ratio the shock velocity decreased with distance in a highly nonlinear manner. The density behind the shock wave was observed to increase significantly before the arrival of the contact surface under all conditions. This surprising shock‐tube behavior is believed to be related to severe laminar boundary layer development behind the shock wave at low initial pressures.