Chemisorbed iodine adlayers on Au(lll) films on quartz were studied using scanning tunneling microscopy (STM) at room temperature in air. The iodine was adsorbed by successive deposition of droplets of a dilute solution of iodine in methanol. As a function of coverage, various adlayer structures were obtained. By changing the tunneling parameters, either the iodine adlayer or the Au(lll) substrate can be imaged with atomic resolution. In this way, the adlattice properties such as periodicity, orientation and the local absolute coverage have been characterized with high accuracy. In the low coverage range (theta<0.33), due to the high mobility of iodine atoms, only the unreconstructed Au(111) substrate lattice could be imaged. At theta similar to 0.33, a (root 3x root 3)R30 degrees structure is evident. With increasing coverage, a (px root 3) structure of higher iodine packing density is observed, which can be described as a uniaxially compressed (striped) phase. Finally, near monolayer saturation coverage the iodine atoms form a hexagonal moire-like pattern with long-range height modulation. The results are compared with previous measurements on this system under UHV and electrolyte conditions. (C) 1997 American Institute of Physics.