Scanning tunneling microscopy (STM) was used to quantatively measure single molecule diffusion at the solution/solid interface. Strong adsorbate-surface interactions allow isolated double-decker phthalocyanine Y[C₆S-Pc]₂ molecules to be observed and tracked on Au(111), both in the presence of solvents and in argon. This work investigates the influence of STM tip, temperature, and environment on surface diffusion through analysis of the molecular trajectories in sequential STM images. The surface diffusion of Y[C₆S-Pc]₂ was found to be thermally activated with almost no motion observed at 5 °C; whereas, above 35 °C molecular motion is so rapid that it becomes difficult to track single molecules. At room temperature, the influence of the STM tip was quantified by varying the sample bias voltage and the diffusion coefficient was found to vary between 0.6 x 10^-17 and 16 x 10^-17 cm/s. The surface diffusion of molecules was also considerably influenced by the nature of the solvent, with motion in argon being too rapid to reliably track with our methods. An important implication of this study is that even in the case of very strong adsorbate-substrate interactions, the STM tip can significantly mobilize surface molecules and thereby enhance the formation of self-assembled films. Moreover, because the tip induced displacements are not unidirectional, one cannot diagnose tip induced motion by analyzing the displacements at one set-point and scan rate. Particular care must be taken in any STM based studies of self-assembly kinetics at the solution-solid interface.