Laser flash photolysis ((exc) = 266 nm) of -sulfinyl diazo compound 1b results in the formation of two transient phenomena. Transient absorption A (intense, (max) = 275 nm) shows jump-and-growth behaviour with a growth lifetime of = 2 s. Transient absorption B (weak, (max) = 415 and 540 nm) decays with a lifetime of = 1.8 s, which suggests that B decays into A. On the basis of a comparison with calculated UV/Vis spectra, transient absorption B is assigned to triplet carbene (3)7. DFT and CCSD(T) calculations indicate that carbene 7 should have a singlet (S) ground state with a singlet-triplet energy gap of ca. 11 kcalmol(-1) in acetonitrile solution. The seemingly paradoxical observation of a triplet excited state of a ground-state singlet carbene is rationalised in terms of the very different geometries of the singlet and triplet carbene; the most prominent difference lies in the pyramidalisation at the sulfoxide sulfur atom. For the singlet carbene (1)7, the O-S-C:-C(=O) dihedral angle is calculated to be 172.8 degrees, whereas it is -80.0 degrees for the triplet carbene (3)7. Therefore, for efficient intersystem crossing (ISC) to occur, the triplet carbene must significantly change its geometry. A triplet-singlet minimum energy crossing point (MECP) was located with an O-S-C:-C(=O) dihedral angle of -141.3 degrees. On the basis of the energy of this MECP (15.2 kcalmol(-1) above the S carbene in CH3CN) and on the ISC rate constant of diphenyl carbene taken as preexponential factor, a rate for the ISC of (3)7 is estimated that matches the experimental value within one order of magnitude.