A remarkably sensltlve cathodic stnppmg voltammetnc procedure for ultratrace measurements of selenmm m the presence of added rhodmm 1s described The method IS based on the accumulation and subsequent reduction of a Rh,Se, layer on the hangmg mercury drop electrode Optmmm expenmental condrtlons were estabhshed, mcludmg the use of a 0 1 M sulfunc acid solution contammg 10 pg 1 -' rhodmm, and preconcentration at -0 2 V followed by a fast bnear scan Such con&tlons yield an extremely low detetion hmrt of 0 5 ng 1-l (6 x 1O-*2 M) selenium followmg a 3-mm preconcentrafion The improved sensltivlty (over earher cathodic smppmg voltammetnc schemes) IS coupled to high select~vlty and preclslon Appbcabtity to assays of groundwater and nver water samples IS illustrated Keywords Strxppmg voltammetry, Rhodwm, Selenmm Trace measurements of selenium have received much attention 111 recent years because of the blologxal slgmficance of this element In particular, the narrow concentration range between deficient and toxic levels of selemum has stimulated the development of ultrasensrtlve methods for Its determmatlon [l] Electroanalytlcal techmques, partxularly stnppmg voltammetry, have been shown by several workers to be extremely useful for tlus task [l-81 While anodlc stnppmg procedures (at gold electrodes) were used m previous years [3], cathodic stnppmg schemes (at the static mercury electrode) have been adapted recently for routme apphcatlons because of the ease of attammg renewable and