Transformations taking place in alkanethiol adlayers on Au(111) in contact with an electrolyte solution in the potential range - 1.1 V < E< - 0.4 V have been investigated by in situ scanning tunneling microscopy (STM). Two different types of processes have been identified: potential-dependent transformations involving electrodesorption/electroadsorption of molecules, and structure and defect fluctuations that take place without the influence of the electric potential. Potential-dependent transformations involve two steps: electrodesorption of chemisorbed alkanethiol molecules from Au(III) terraces producing unstable physisorbed aggregates of alkanethiolates, and electrode sorption of alkanethiol molecules that remained chemisorbed at step edges due to the increased coordination number at these substrate sites.