A new improved fluorescence correlation spectroscopy method for determining motional states of particles, fluorescence correlation spectroscopy with traveling interference fringe excitation (FCSTFE), is described. With this method the modulated fluorescence signal from particles excited by an interference fringe moving at constant velocity is detected, and cosine and sine Fourier coefficients at the frequency of the traveling fringe [Fcos(t),Fsin(t)] are recorded. Autocorrelation functions ofFcos(t),Fsin(t), and the square of amplitude [A2(t)=Fcos(t)2+Fsin(t)2] were found to express the motion of the fluorescent particles. The apparatus utilizing a photon counting system was constructed, and the performance of the system was demonstrated by the determination of diffusion coefficients of dispersed fluorescent polystyrene spheres. Features of FCSTFE in particle dynamics evaluation are discussed. ©1996 American Institute of Physics.