A conductimetric study of foam formed from mixture of the protein, ${eta}-lactoglobulin$, and the nonioinc surfactant, SML, revealed that their stability was reduced at concentrations of SML in the range $3{sim}10mM$. The interaction of SML with ${eta}-lactoglobulin$ was investigated by fluorimetry and a dissociation constant of $0.2{mu}M$ was calculated for the complex. Surface tension studies confirmed the presence of interaction between the two components and provided evidence for the progressive displacement of ${eta}-lactogloblin$ from the air/water interface with increasing SML concentration. Experiments using air-suspended microscopic thin liquid films revealed transitions in the chainage characteristics and thickness of the film at SML concentrations below that which resulted in destabilization of the foam. However, measurements of surface mobility of fluorescent-labeled ${eta}-lactoglobulin$ by a photobleaching method identified that a transition to a mobile system occurred at a SML concentration which correlated with the onset of instability in the disperse phase. The results would indicate that maintenance of the viscoelastic properties of the surface is paramount importance in determining the stability of interfaces comprising mixtures of protein and surfactant.