A newly developed strategy based on gold (Au) electrode modified with cadmium sulfide nanoparticles (CdSnp) and thionine (Th) was proposed toward electrocatalytic and photoelectrocatalytic hydrogen peroxide (H2O2) reduction. At first, a thin film of CdS nanoparticles was electrodeposited onto Au electrode. Then, the CdS/Au electrode was modified with mercaptoacetic acid (MAA), which not only acts as a stabilizing agent to prevent the chalcogenide CdS nanocrystals from aggregation but also as a linker for subsequent attachment of Th onto the CdS nanoparticles. The effective covalent immobilization of Th was achieved through amide bond formation reaction between -NH2 groups of Th and -COOH groups of MAA, using dicyclohexylcarbodiimide (DCC) as condensation agent. The Au/CdS/Th modified electrode showed a well-defined redox couple with surface confined characteristics at wide pH range (2-12). The heterogeneous electron transfer rate constant (k(s)) and the surface coverage of immobilized Th on the modified electrode was obtained as 0.12 s(-1) and 4.35 x 10(-9) mole cm(-2), respectively. The electrocatalytic activity and stability of the modified electrode toward hydrogen peroxide reduction was investigated and it was found that the Au/CdS/Th electrode illustrates excellent electrocatalytic activity toward H2O2 reduction at reduced overpotential. The detection limit, sensitivity and catalytic rate constant (k(cat)) of the modified electrode toward H2O2 were 55 nM, 3.4 mu A mu M-1 cm(-2) and 3.75 (+/- 0.1) x 10(3) M-1 s(-1), respectively, at linear concentration range up to 10 mM. Upon light irradiation, about two-fold improvements were attained in sensitivity and detection limit of the modified electrode toward H2O2 electrocatalytic determination. (c) 2013 Elsevier Ltd. All rights reserved.