Using a confocal fluorescence microscope, we found a significant temporal fluctuation in the fluorescence spectrum of porphine molecules on the water surface. The spectra fluctuated when the accumulation time was 300 ms and the pH was within a certain range. No fluctuation was observed with a longer accumulation time (36 s) or on a solid surface. The present observation indicates that the distribution of porphine on the water surface is heterogeneous and that the Brownian motion of the surface species induces spectral fluctuation. A neutral form of porphine produces domains at a higher surface coverage.