Composites of bismuth and gold nanoparticles (Bi–AuNPs) were prepared on a carbon paste electrode (CPE) by an electrochemical deposition method. The electrochemical parameters were optimized for the effective deposition of Bi–AuNP composite materials. Microscopic examination revealed that nanoparticle clusters of gold were embedded well within the bismuth films (Bi films) over the surface of the CPE. An analytical anodic stripping voltammetry method has been developed for the simultaneous determination of copper and mercury at ultratrace levels using the Bi–AuNP modified CPE. It was possible to observe well separated stripping peaks of the two metal ions and the modified electrode was successfully applied for simultaneous determination of Cu and Hg. The limits of detection using the optimized analytical procedure were observed as 0.16 μg L⁻¹ and 0.28 μg L⁻¹ for Cu and Hg respectively. Interference effects of some of the commonly occurring metal ions were investigated and the method was applied for the determination of Cu and Hg in two ground water and two soil samples collected from different places.