The construction of a colloidal gold-cysteamine-carbon paste electrode, Au(coll)-Cyst-CPE, for the electrochemical determination of homocysteine is reported. The improved voltammetric behaviour of homocysteine at Au(coll)-Cyst-CPE with respect to that observed at a gold disk electrode is attributed to an enhanced electron transfer kinetics as a consequence of the array distribution of gold nanoparticles immobilized onto the Cyst SAM. Cyclic voltammetry of homocysteine showed an adsorption-controlled current for scan rates between 500 and 5000 mV s(-1). The hydrodynamic voltammogram constructed for homocysteine allowed the selection of a potential value of +600 mV, where the background current is negligible, for the amperometric detection of the analyte at the Au(coll)-Cyst-CPE. Using a flow rate of 0.8 ml min(-1), the R.S.D. value for i(p) after 25 repetitive injections of homocysteine was of 4.3%, and one single electrode could be used for more than 15 days without any treatment or regeneration procedure of the modified electrode surface. An HPLC method for the separation and quantification of homocysteine and related thiols, using amperometric detection at the modified electrode has been developed. A mobile phase consisting of 2:98% (v/v) acetonitrile:0.05 mol l(-1) buffer solution of pH 2.0, and a detection potential of +0.80 V were selected. Separation with baseline resolution and retention times of 3.00, 3.60, 4.52, 5.71 and 7.79 min were obtained for cysteine, homocysteine, glutathione, penicillamine and N-acetyl-cysteine, respectively. Calibration graphs were constructed for all the separated compounds. Detection limits ranged between 20 nM for cysteine and 120 nM for penicillamine, with a value for homocysteine of 30 nM. These values compare advantageously with those achieved with previously reported HPLC methods using electrochemical, UV, fluorescence and MS detection modes. The developed method was applied to the determination of cysteine and homocysteine serum samples with good results.