Carbon-13 n.m.r. spectra were obtained for eight compounds having C = Se double bonds. These comprised five selenoketones, 2,2,4,4-tetramethylpentane-2-selone (1-Se), 1-thia-3,3,5,5-tetramethylcyclohexane-4-selone (2-Se), 1,1,3,3-tetra methylindane-2-selone (3-Se), 2,2,5,5-tetra methylcyclopent-3-ene-1-selone (4-Se), and selenofenchone (5-Se), one selenoester, ethyl selenobenzoate (6-Se), and two selenoamides, NN-dimethylselenobenzamide (7-Se) and NN-dimethyl-2,2-dimethylselenopropanamide (8-Se). Coupling constants, 1J(¹³C–⁷⁷Se), were measured for (1) and (3)–(7). A comparison was made among the chemical shifts of (1)–(8), their oxygen analogues, and the sulphur analogues of (1), (5), and (7). The selenocarbonyl carbon atoms were shifted 34–72 p.p.m. downfield and the α-carbon atoms 10–19 p.p.m. downfield from their oxygen analogues. The differences, δΔ_{Se, O}, for the remaining carbons are only a few p.p.m. Values of ¹J(¹³C–⁷⁷Se) were 209–221 Hz, much larger than any previously observed ¹³C–⁷⁷Se coupling constants.