Bovine serum albumin (BSA) was reacted with linear and newly synthesized branched oligothiophene N-hydroxysuccinimidyl ester fluorophores (TSEs) in moderately basic carbonate buffer solution. Optically stable BSA-TSE conjugates were obtained with a degree of labeling depending on experimental conditions. Conjugates with high fluorophore to BSA ratios (F/BSA = 8) displayed fluorescence quantum yields in the range of 10-30% in water at pH = 7.2, comparable to the quantum yield (25%) of the BSA-FITC conjugate prepared under the same conditions and with the same degree of labeling.