Reaction of hydrogen selenide with l-methyl (ene)-1,3-diones in ethanolic hydrogen chloride containing iron(III) chloride affords either bis(3-methyl(ene)-1,2-diselenolium)tetrachloroferrates (II) or tris(1-niethyl(ene)-1,3-diselenato)iron(II) complexes, and 2,4,6,8-tetraselenaadamantanes, Bis(3-methyl (ene)-1,2-diselenelium) tetrachloroferrates (II) are readily converted into the corresponding perchlorates. Dimethylthipformamide condenses with both 3 methyl(ene)-1,2-diselenolium perchlorates and tris(l-methyl (ene)-l,-3-diselenato) iron (III) complexes to give 3-(2-dimethylaminovinyl)-1,2-diseleolium (Vilsmeier) salts, These salts react with sodium hydroxide to form 3-acylmethylne-3H-l,2-dise1enoles, with sodium hydrogen sulphide to give 1-thia-6,6a-dise1napentalene, and with sodium hydrogen selenide, to form 1,6, 6a-diselenapentalenes, and with sodium hydrogen selenide to fom 1, 6, 6a-triselenapentalenes. In addition, treatment of 3-(2-dimethylamino-1-methylvinyl)-4-methy1-1, 2-dislenolium percholorate with sodium hydroxide, sodium hydrogen sulphide, and sodium hydrogen selenide affords 3, 5-dimethyl-4H-pyran-4-selenoketpne, 3, 5-dimethyl- 4H-thopyran-4-seleoacetone, and 3, 5-Dimthyl-4H-slenopyran-4-selenoketone respectively. 3, 5Dimethyl-4H-selenopyran-4-selotone is hydrolysed to 3,5-dimethyl-4H-selenopyrap-4-on in wet chloroform solutton. As a dry crystalline solid, 3,5-dimethyl-4Hr-selemopyrao-4-selenoketone is a tmospherically oxidised to 3, 5-dimethyl-4H-selenopyran-4-one, 3-(l- formylethylidene)-4-methyl-3H-l,2-diselenole, and 3,4-dmethyl-1,6-dioxa-6a-selenapnta1ene, 3,4-Dimthyl-l,6,6a-triselenapentalene and 4,5-dihydro-3H-bnzo[cd)]. 1,6,6a-trisolonapenalene are formed by treatment of 3-(l-formylethy1idene)-4-methyl-3H,1-2-diselenole and 3-formyl-5,6-dihydro-411-bonzo[c]1,2-diselenole respectively with phosphoryl chloride in dpiethylformamide and subsequent addition f aqueous potassium selenosulphate or potassium selenotrithionate. Compounds containing oxygen and selenium atoms in the 1- and 6, positons of the 6a-selenapentalene ring, when treated with phosphorus pentasulpc.1e, form 1,6-dithta-6a-selenapentalnes provided the 2- and 5-positions are unsubstituted. The structures of 3-acylmethylene-3H-l,2-diselenoles and 1-thia-6,6a-diselenapontalenes are discussed in relation to spectroscopic data and compared with those of 3-acymetbylene-3H-l,2dithioes and 6a-thiathiophthepo respectively, The crystal structures of 6a-thiathiophthene, 1,6.dftia-6a-selenapentalene, and 1 ,6,6a-triselnapentalene are compared and the structures of l,6-dithia-6a-selepapeptalenes and 1,6,6a-triselenapentlenes are discussed in relation to spectroscopic data, A bicyclic structure containing an oygen-selenium-oxygen bonding sequence is proposed for 3,4-dimethyl-1,6dipxa6a-selenapentaleqe on the basis of its N.M.R and I.R, spectra. The mechanism of the rearrangement of 6a-thiathiopbthenes by aqueous sodium sulphide to 4H-thiopyran-4-thiones has been studied with sodium sulphide-S-35. The initial step in the rearrangement is a reductive cleavage of sulphur-sulphur bond forming an anion which is then susceptible to nucleophilic attack by sulphide ion at the 2- (and/or 5-) position(s).