To stabilize Hg metal (i.e. to precipitate HgS) In a thermally pretreated spent (brine electrolysis) comminuted C electrode (I) and In an active C-borne spent (vinyl chloride process) catalyst (II), containing 759-791 and 245-300 mug of H2O-leachable Hg/g I and g II, resp., each waste, 1 g, was treated with (i) 2 mL aq. (NH4)(2)S, (ii) 0.5 g K2Sx (+ 2 mL H2O), (iii) 0.5 g Na2S (+ 2 mL H2O)1 (iv) 2 mL aq. satd. CaSx at room temp. for 30 days. To study the Hg stabilization effect, the sulfide-treated I and II were leached each with 10 mL H2O for a total of 27 h, and in 3 successive extracts Hg was determined by cold-vapor AAS. With II, (i) and (iii) were best as Hg Immobilizers, allowing only 2 and 8 mug Hg/g II, resp., to be released. With I, no immobilizer was effective: in 3 extracts, I released In total 239-816 mug Hg/g. Unlike crude I, the S-treated I released Hg Into water in descending amounts.