The foam stability of aqueous solutions of pentaethylene glycol n-decyl ether and pentaethylene glycol n-dodecyl ether surfactants was related to experimental studies carried out on thin horizontal microscopic single films of the surfactant solutions using the microinterferometric film apparatus developed by Scheludko [1]. It was observed that an initial decrease in film thickness and reduction in potential at the air/solution interface of the film, occurred at a critical surfactant concentration where "black spots" appeared in the film when it was observed in reflected light. The black spots appeared to expand and coalesce, until the entire sheet was thinned down to a metastable common black film. In addition, a sharp increase in foam stability occured above this surfactant concentration, which was about 0.1 c. m. c, and corresponded to a high surface adsorption density of surfactant. We believe that the cause of the phenomenon was a change in state of the surfactant monolayer as originally proposed by Scheludko and Exerowa [2, 3]. Such a change (condensation) from gaseous to liquid expanded monolayers, has already been established for ionic surfactants and may change the rheological properties of the adsorbed film, enhancing the foam stability. Finally, the foam stability was determined at a range of electrolyte (KCI) concentrations from which it could be suggested that two types offoam stabilization methods were operating In the low electrolyte concentration, the stability appeared to be controlled by the metastable common first black film, but at high electrolyte concentrations the stability seemed to be dependent on secondary "Newton black" films