The isotropic response Of CS2 has been observed by polarisation-resolved ultrafast Raman spectroscopy. The effects of dilution and dispersion in a microemulsion are investigated. The data are contrasted with measurements of the anisotropic response. Previously noted similarities in the relaxation dynamics of the two responses are not preserved on dilution. A simple collision-induced model successfully reproduces the form and timescale of the dynamics, but not the influence of dilution. In all cases the isotropic response of the microemulsion differs from that of the liquid, suggesting the technique has potential to investigate dynamics in complex fluids. (C) 2003 Elsevier Science B.V. All rights reserved.