The photophysical properties and isomerization of 3,3′-diethylthiacarbocyanine (DTC) were studied in isooctane/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/water reverse micelles and in aqueous solutions containing sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (CTAB) and chloride (CTAC) and Triton X-100 (TX100) at concentrations below and above the critical micelle concentrations (c.m.c.). The absorption and fluorescence spectra of DTC in AOT reverse micelles are red-shifted and the nonradiative deactivation rate constant decreases strongly relative to that in water, leading to higher fluorescence quantum yields (Φ_f) and lifetimes. The DTC fluorescence decay in AOT reverse micelles and TX100 normal micelles is biexponential. SDS induces aggregation of DTC in water at concentrations well below the c.m.c. (10⁻⁵–10⁻³ mol l⁻¹). Addition of SDS and TX100 at concentrations just above the c.m.c. increases Φ_f steeply, while CTAB (or CTAC) exerts such an effect only at concentrations much higher than the c.m.c. A rise in the isomerization activation energy and the pre-exponent is observed in the reverse micelles with small w₀ (w₀ = [water]/[AOT]); the isomerization rate constant decreases in SDS, TX100 and CTAC micelles.