Transmission fluctuation spectrometry (TFS) is a method for particle analysis based on the statistical fluctuations of the transmission signal when the particle system is illuminated by a beam with finite beam diameter. The expectancy of the transmission square (ETS) is obtained as a spectrum when the signal is subjected to a procedure of variable spatial and/or temporal averaging. Both the particle size distribution (PSD) and particle concentration can be retrieved. In this paper, we develop a new method of the spectral analysis on the transmission fluctuations. An analytical expression of the spectrum of the fluctuating transmission through a monolayer is derived and the transmission fluctuation spectrum of a 3-dimensional system is formulated based on the layer model with an assumption of low particle concentration. Theoretical results are proved by simulations and measurements. (c) 2006 Elsevier B.V. All rights reserved.