Photoinduced electron transfer (PET) has been investigated in organized monolayers at the air-water interface and in monolayer assemblies on glass in an effort to evaluate the influence of solvent reorganization and molecular dynamics on PET. The donor monolayer contained an amphiphilic thiacyanine dye, and the electron acceptors were methyl viologen and dioctadecyl viologen, respectively. The distance dependence is described here by a hard disk model, where an acceptor molecule within a disk with a radius r(DA) around the excited donor molecule quenches the donor fluorescence due to electron transfer. Acceptor molecules outside the disk are considered ineffective. The critical radius r(DA) is larger in monolayer assemblies on glass (r(DA) = 1.97 nm) than at the air-water interface (r(DA) = 1.15 nm) as evaluated from steady-state fluorescence quenching. This large difference indicates that the time between thermal collisions generating and destroying the energetic match required for electron tunneling from the excited donor molecule to the acceptor is quite different in the two systems that are compared.