Elemental sulfur reacts with ketones and gaseous ammonia at room temperature yielding thiazoline-3 in excellent yields. Under the same conditions elemental selenium does not react at all. Attempts using the known techniques which have been applied in improving the reactivity of slowly reacting ketones in thiazoline-3-synthesis have been unsuccessful (Exp. No. 1–17). The reaction of α-halogenketones with sodiumhydrogenselenide to synthesize α-hydroselenoketones gives only the original ketones and selenium in almost quantitative yields (No. 18–23). The same is observed with α-haloketones and sodium ore magnesium-diselenides (No. 24–45). The explanation of these unexpected results is the strong reducing power of the hydrogenselenide (No. 46–52). Even α-bromoketones with activated bromine (i.g.by phenyl groups) were reduced by sodiumhydrogensulfide giving red undefinite oils. However, α-chloroketones give α-mercaptoketones in excellent yields (No. 53–61). Hydrogenselenide reduces α-mercaptoketones to sulfur and ketones in the presence of triethylamine (No. 62–67). Also the transformation of α-selenocyanketones to α,α′-diketodisenides by alkali or the oxidative hydrolysis of selenium-“BUNTE” salts does not work and gives elementary selenium only. Studies about the concomitant reaction of elementary selenium and ethylenimine on ketones