The monolayer characteristics of the four surface-active derivatives of crown ethers (I-IV) with different ring size were studied under various conditions. The area per crown ether molecule in monolayers at the air-water interface increases with increasing size of the polyether ring, whereas the surface potentials remain almost the same. The areas per molecule are increasing significantly in the presence of various alkali metal cations in the aqueous subphase, especially in the case of K+ .The highest absolute value was fouund for crown ether III with six oxygen atoms in the polyether ring. The surface potential values for all studied crown ethers decrease in the presence of the alkali metal cations. These effects can be explained as complex formation between crown ethers in the monolayers and cations from the aqueous subphase. Using the recently developed Brewster angle microscopy, the differences of the monolayer morphology in the presence and absence of the alkali metal cations in the aqueous subphase were monitored, providing additional evidence for complex formation.