Detailed degenerate four wave mixing (DFWM) studies of four molybdenum complexes, cis-Mo(CO)4 (PPh3)2, cis-Mo(CO)4(AsPh3)2, cis-Mo(CO)4(PPh2Np)2 and cis-Mo(CO)4(P(OPh)3)2 are reported in this paper. Upon exposure to air/oxygen, the χ(3) values of solutions of the first three complexes increase dramatically with time. However, when excess free ligand is added to any of the solutions, no additional increase in the χ(3) value is observed. Hence, it is possible to obtain solutions with high χ(3) values by first oxidizing the solution and then adding excess ligand to stabilize the solution. Further, the increase in the χ(3) values of the solutions parallels the increase in the linear absorption, but it is unclear whether the increased linear absorption plays any role in the increase in the χ(3) values of the solutions. This study also shows how degenerate four wave mixing can be used as a very sensitive technique to detect the chemical change in a solution when the third order optical nonlinearities of the reactant and product are very different. © 1998 Published by Elsevier Science B.V. All rights reserved.