The atmospherically relevant reactions between dimethyl selenide (DMSe) and the molecular halogens (X-2 = Cl-2, Br-2, and I-2) have been studied with ab initio calculations at the MP2/aug-cc-pVDZ level of theory. Geometry optimization calculations showed that the reactions proceed from the reagents to the products (CH3SeCH2X + MX) via three minima, a van der Waals adduct (DMSe:X-2), a covalently bound intermediate (DMSeX2), and a product-like complex (CH3SeCH2X:HX). The computed potential energy surfaces are used to predict what molecular species are likely to be observed in spectroscopic experiments such as gas-phase photoelectron spectroscopy and infrared matrix isolation spectroscopy. It is concluded that, for the reactions of DMSe with CI2 and Br-2, the covalent intermediate should be seen in spectroscopic experiments, whereas, in the DMSe + I-2 reaction, the van der Waals adduct DMSe:I-2 should be observed. Comparison is made with previous related calculations and experiments on dimethyl sulfide (DMS) with molecular halogens. The relevance of the results to atmospheric chemistry is discussed. The DMSeX2 and DMSe:X-2 intermediates are likely to be reservoirs of molecular halogens in the atmosphere which will lead on photolysis to ozone depletion.