An experiment performed on INTER last year found extreme non ideal mixing between C12E8/SLES3 at the air water interface: this feature became more extreme as the temperature dropped from 25oC to 10oC. The promotion of C12E8 to the interface was much higher than previously expected. We wish to see if this effect is observed in other non ionic/anionic surfactant mixtures by studying C12E8/LAS4. LAS4 is much more surface active when compared to SLES3 because it has a branched alkyl chain, which increase its hydrophobicity. A comparison between the two systems will help characterise what is driving the increased adsorption.