Photoisomerization of 3,3＇-diethyloxadicarbocyanine iodide (DODCI) between the id and P isomers at bare borosilicate and octadecyl chain-modified C-18 glass/ethanol interfaces was studied by steady-state and time-resolved total-internal-reflection (TIR) fluorescence spectroscopies. Both the fluorescence spectrum and the lifetime of DODCI determined at the glass/ethanol interface were different from those in bulk ethanol. Simulations of the photoisomerization kinetics on the basis of a three-valley S-1 potential surface diagram revealed that the kinetics of P --> N isomerization in the ground state was largely affected by the presence of the octadecyl groups on the glass surface, while that of N --> P conversion in the excited state was similar but slightly decreased compared to that in a bulk phase. The large decrease in the P --> N isomerization rate was explained in terms of an increase in local friction (viscosity) at the interface and strong structural interactions between DODCI and the octadecyl chains bound to the glass surface.