The photochemistry of OCSe with ClF trapped together in argon matrices at cryogenic temperatures has been explored and the first interhalogen representatives of the elusive XC(O)SeY family, namely syn-ClC(O)SeF, anti-ClC(O)SeF, syn-FC(O)SeCl, and anti-FC(O)SeCl, as well as the hitherto triatomic species ClSeF complexed by a CO molecule, were obtained. Both ClC(O)SeF conformers appear to be produced independently by photolysis of the respective precursors; while formation of both FC(O)SeCl structures is ruled by the presence of an angular molecular complex OCSe center dot center dot center dot Cl-F formed prior to photolysis. This latter photochemical pathway seems to favor the formation of the less stable anti-FC(O)SeCl structure instead of the more stable syn- one. With the aid of quantum chemical calculations, using ab initio, DFT, TDDFT, and CASSCF methods, the mechanism for this photochemical reaction is rationalized both in terms of radical processes as well as a photoinduced electron transfer occurring into the OCSe center dot center dot center dot Cl-F complex. Also a singlet-triplet conical intersection between anti and syn rotamers of the FC(O)SeCl molecule is theoretically explored.