An atomic force microscope (AFM) study of the structure and stability of islands of alkanethiol (C16H33SH) grown on Au(111) is presented. Ripening processes involving long-range molecular diffusion takes place at ambient temperature in the scale of days. In the partially covered surface, ripening is manifested by an enlargement of the islands as well as by molecular densification. On the well packed islands, the c(4x2) superstructure is imaged. Another manifestation of island densification is a substantial increase in mechanical stability, as shown by the resistance of the film to penetration by the tip. The threshold pressure for molecular displacement and tip penetration was found to increase by almost 70% relative to the initial value before ripening.