Third-order nonlinear optical responses of nanoparticulate Co3O4 films, prepared by sputtering and pyrolysis, were investigated. The sputtered Co3O4 film showed a large third-order nonlinear susceptibility (\chi((3))\) of 3.2 X 10(-8) esu, determined by degenerate four-wave mixing (DFWM). This \chi((3))\ is 7.1 times greater than that of the pyrolyzed Co3O4 film, attributable to the greater density and refractive index of the sputtered film. Time-resolved DFWM experiments show a slowly decaying component with a lifetime of approximately 100 ps existing for both films in addition to a fast decaying component. Nonlinear transmission experiments revealed both films exhibiting significant nonlinear absorption. The imaginary part of the susceptibility (Im[chi((3))]) found for each film was of the same order of magnitude as the \chi((3))\ found by the DFWM method, showing that the major part of the fast decaying component of \chi((3))\ comes from the nonlinear absorption. (C) 2003 Elsevier B.V. All rights reserved.