Experimental values of the wave length and wave velocity have been obtained far dilute sohtions of valeric and hexanoic acid for a vertical fulling liquid film. The wave length was unaffected by the surfactants for Reynolds numbers in the range 5 to 100; however, the wave velocity was decreased for increased surface concentrations of the two acids. This is in direct contradiction to previous theoretical work, and the explanation for the anomaly is that the free surface velocity is greatly retarded by the adsorption of the surface active agents.In an effort to determine the extent of this retardation, an approximate form of the diffusion equation and the equations of motion were solved subject to boundary conditions describing the effect of the adsorbed surfactant on the surface stress. The results indicate that the entrance length can be increased several orders of magnitude by the addition of small amounts of surfactant.The flow characteristics of falling liquid films have been a subject of interest in connection with gas absorption studies (9, 17, 24, 27) primarily because the falling film appears to provide a reasonably well defined interfacial area and contact time. Knowledge of the nature of the flow of thin liquid films is also of importance in analyzing heat transfer rates for ablating space vehicles ( 3 1 ) , in the design of cooling systems for high temperature turbine blades (15), and in the analysis of mass transfer rates in certain desalinization processes ( 6 ) .By itself, the falling liquid film provides a reasonable experimental tool for studying the effect of surface active agents on bulk fluid motion. In addition to the wave length and wave velocity, one can also observe the location (measured from the top of the film) of the first visible waves as a function of Reynolds number, and the rate of approach of the surface velocity to the value predicted for uniform flow.In order to test recent mathematical studies (1, 4, 28, 29) of the stability of vertical falling liquid films, an experimental study of the wave length and wave velocity was planned. In a sense, the experimental study was unsuccessful for the data were obtained under conditions which did not allow the direct comparison of theory and experiment; however, some interesting characteristics of the flow were observed and analyzed, and it seems appropriate to report the results at this time.