A flow visualization technique based on photochromic dyes has been used for the study of polymeric fluids in a number of flow configurations. The technique is based on the activation of the dye by ultraviolet light. This photoactivation results in the apparition of dark marker lines in the midst of the fluid. High‐speed movies can then be digitized and allow us to compute velocities anywhere inside the flow field. The results presented here address more particularly the case of the tubeless siphon. We have measured the velocity profile inside its fluid column for a number of flow conditions, and observed unexpectedly large velocity gradients. Indeed, the axial velocity at the centerline was found to be much greater than the velocity of the free surface at the same axial location. An interesting feature of this flow field is that the velocity profile appears to be a linear function of the radial location over most of the cross‐section of the column.