The photochemistry of 4,5-carbomethoxy-1,2,3-thiadiazole in solution was studied at room temperature with use of UV-vis and IR transient absorption spectroscopies (lambda(ex) = 266 nm). Ultrafast time-resolved techniques demonstrate that there is a very fast rise (<0.4 ps) of bis(carbomethoxy)thiirene in acetonitrile, and that it is the only intermediate formed. The lifetime of the thiirene is limited by dimerization to eventually form tetra(carbomethoxy)thiophene.