Insoluble monolayers at the air-water interface are exposed to a gas phase containing organic hydrocarbons. The hydrocarbons are partly incorporated within the monolayer which leads to changes in orientational order and the formation of new phases of different morphology. The transition state resembles features of the air-water and oil-water interface and the control of the hydrocarbon partial pressure allows continous tuning between both interfaces, The phospholipid D,L-alpha-dipalmitoylphosphatidylethanolamine, DPPE, and an esterdiol hexadecanoic acid, 2,3-dihydroxypropyl ester, ESD-16, are used as amphiphiles, and pentane, n-hexane, cyclohexane, 2,2-dimethylbutane, n-heptane, n-decane, and n-dodecane are used as hydrocarbons. Both amphiphiles differ in their headgroup size. In DPPE the aliphatic tail determines the packing within the monolayer, but in the case of ESD-16 it is the headgroups. The structural changes are monitiored by surface pressure-area (pi,A) isotherms and imaging ellipsometry. The influence of the chemical nature of the hydrocarbon and the effect of the partial pressure of the hydrocarbon on the monolayer structure are assessed.