The reaction of 1,2 ethanedithiol (EDT) with selenous acid in water or alcohol leads to selenopolysulfide chains or cycles, (C2H4SSeSC2H4SS)(n), with randomly distributed -SSeS-and -SS-moieties. The reaction in water produces incompletely reacted material, which on recrystallization, gives an oligomer corresponding to 5 EDT units (pentamer) as confirmed by molecular mass determination, Se analysis, H-1-and Se-77-NMR spectroscopy. In both the pentamer and cyclic forms the incidence of neighboring -SSeS-moieties is higher than that expected statistically. The mechanism for the reaction of thiols with selenous acid provides some rationalization for this observation in as much as neighboring -SSeS-groups, or groups that will lead rapidly to neighboring -SSeS-groups are formed in general before -SS-links can be formed. The Raman spectrum of these products show typical strong SS, SeS, and CS stretching bands at 510, 370, and 730 cm(-1). The high frequency of upsilon(cs) is attributed to a preferred gauche conformation at the CS bonds. For comparison, polydisulfides were also prepared from EDT and iodine in methanol. These products consist of at least seven cyclic polymers ranging from the four-membered 1,2-dithietane to higher members. Heating above 100 degrees C in chloroform for several hours gives a solution containing the four lowest molecular mass rings, which on standing for 24 h, precipitate highly insoluble material, which is probably chain or large-ring polymer. Molecular mass determination in camphor indicates that, like yellow sulphur, chain polymers are formed at the melting point of camphor (170 degrees C).