We have developed a novel technique which utilizes the intensity‐dependent dispersion relation of surface plasmons to study the third order nonlinear susceptibility coefficient &khgr;(3). This new technique yields information about both the magnitude and the sign (with respect to &khgr;(1)) of &khgr;(3). Experimental results are reported for GaAs and Si at input wavelengths of 1.05 and 1.11 &mgr;m, respectively.