Adlayer structures of Cl, Br, and I on the Cu(lll) surface have been investigated using in-situ scanning tunneling microscopy (STM) in solution and ex-situ low-energy electron diffraction (LEED) in ultrahigh vacuum. The atomic structure of Cu(111)-(1 x 1) was observed by in-situ STM in a perchloric acid solution. Complementary use of LEED and in-situ STM revealed (p x root 3) centered-rectangular phases for Cl and Br, in which the structures are compressed almost linearly with potential. In the case of iodine monolayer, a (root 3 x root 3)R30 degrees structure was consistently observed in the potential range investigated. The compression in the adlayers of Cl and Br on Cu(111) can be explained in terms of the difference in their van der Waals diameters.