This report details the measurement of the dependence of the electric field magnitude in the electrical double layer on applied potential for a metal/organic thin film/electrolyte system. The self-assembled monolayers (SAMs) used in these experiments were formed from n-alkyl thiols on roughened Ag and roughened or smooth Au electrodes. The electric field magnitudes were calculated from the Stark shifts of a cationic fluorescent probe, an (aminostyryl)pyridinium derivative, that was immobilized in the SAM. Attention is focused on two gold-supported SAMs, one in which the probe was embedded within the SAM, and another in which the probe was external to the SAM. The composition of the monolayers and the orientation of the probe and n-alkyl thiol molecules within them were characterized using ex situ reflection-absorption Fourier transform infrared spectroscopy (RA-FTIR). For the case in which the probe was embedded within a dodecane thiol SAM, the change in electric field magnitude was determined to be 3 x 10(5) V cm(-1) per volt change in the applied potential, while for the case of the probe outside of a propyl thiol SAM a value of 4 x 10(4) V cm(-1) per volt change in the applied potential was obtained. These values are considerably smaller than expectations based on existing models for the potential distributions within such structures, and possible reasons for this behavior are discussed. The fluorescence intensity from the probe was observed to depend on potential, an effect that is also discussed. Measurements of the Stark shift versus applied potential at different ionic strengths provide a value of the potential of zero charge for these SAMs of -0.4 V versus SCE, in good agreement with previously measured values. The conditions under which fluorescent probes can be used effectively at metal electrodes are also discussed.