We present a new method to decompose the stellar population mixtures in galaxies. The method combines the spectra of simple stellar populations (SSP) of different ages and metallicities, computed with the recently published high spectral resolution evolutionary synthesis models of Bruzual & Charlot (2003). Statistical mechanics techniques, based on Markov Chain Monte Carlo methods, are used in the search for the combination of SSPs which best matches an observed spectrum. The spectral base covers 10 ages and 3 metallicities. The method provides the Star Formation History (SFH) expressed in a population vector $\vec{x}$, whose components represent the fractional contribution of each SSP to the total flux (or mass). The code also estimates the velocity dispersion σ[sstarf ]. A series of simulations were performed to evaluate the code's ability to recover the input parameters (SFH, extinction and velocity dispersion), to investigate the effects of noise in the data and map intrinsic degeneracies of stellar populations.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html