The complex Co(tacn)(NCS)(3) (tacn = 1,4,7-triazacyelononane) has been prepared and characterized by elemental analysis, NMR, IR, and W-vis spectroscopy, and conductivity measurements. It is thermally stable in dmso and aqueous/dmso solution but, on irradiation of the latter solutions at 360 nm, undergoes parallel photosubstitution processes to form dmso- and aqua-substituted products with a quantum yield of 0.012 +/- 0.005. Under these conditions, no photoredox reaction was detected. On addition of thiocyanate ion to the solutions, the product yields were increased in an almost linear fashion. For 1 M thiocyanate solution, the quantum yield for disappearance of the starting complex rose to 0.022 +/- 0.002 and a small redox yield of 0.0008 +/- 0.0003 was found. Under these same conditions, nanosecond laser flash photolysis at 355 nm revealed a transient absorption owing to (NCS)2(-.), which was produced with an estimated quantum yield of 0.036. These results were interpreted in terms of scavenging of radical pair species by thiocyanate ion followed by back electron transfer to give photosubstituted product. Comparison with results for other ammine thiocyanate systems shows that the distribution of redox versus substitution products depends on the rate of loss of the strong field ligands from the Co(II) fragment of the radical pair. A radical pair yield of 0.29 and lifetime of 12 ps was derived from a kinetic model of the scavenging by thiocyanate. The final recombination step between the Co(II) fragment and (NCS)2(-.) was shown to be second-order and, hence, that it was a bulk recombination process with k = 6 x 10(9) M(-1) s(-1).