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The dilatational rheology of adsorbed films of β-lactoglobulin has been studied at the air–aqueous solution interface and the n -tetradecane–aqueous interface using a Langmuir trough apparatus employing a pulse change in interfacial area. The effect of the addition of the non-ionic surfactant C 12 E 6 to films adsorbed from 10 −3 wt% β-lactoglobulin at pH 7 and 30°C was also examined. The surfactant appears to bind to β-lactoglobulin with a low molar ratio (one surfactant molecule per protein molecule), but this and/or co-adsorption has significant effects on the dilatational moduli. Films exhibit higher dilatational moduli at the oil–water interface compared to the air–water interface and show a stronger dependence on the surfactant concentration at the oil–water interface. At around equimolar ratios of protein and surfactant there is a slight maximum in the dilatational moduli, more noticeable at the oil–water interface, though separate measurements also indicated a slight enhancement of foam stability in this region. At higher surfactant concentrations there is a more marked decrease in the dilatational moduli to similar values for both types of interface. Moduli measured at strains up to at least 5% appear to be considerably non-linear.