Fluorescence decay dynamics and fluorescence anisotropy of N,N'-dioctadecylrhodamine B (DORB) at the air/water interface with surface molecular areas of similar to 10(5) Angstrom(2)/molecule were examined by picosecond single photon timing spectroscopy. The interfacial dielectric constant around DORB was estimated to be similar to 21 by the dependence of nonradiative rate constant on polarity parameter E-T(30). The rotational relaxation of DORB at the air/water interface showed two-exponential behavior with lifetimes of 3.15 +/-0.14 ns and 420 +/- 30 ps. The longer component was interpreted in terms of the formation of a DORB cluster, and the average number of molecules in clusters was estimated to be 7 similar to 8.