A computational algorithm, capable to simulate RBS spectra recorded during scanning proton microprobe analysis of organic specimens with inhomogeneous thickness has been developed. This program is based on an enhancement of the RUMP source code. It allows us to normalize in terms of dry specimen mass the elemental concentrations obtained with simultaneous PIXE and RBS microanalysis. This code includes non-Rutherford cross sections for proton scattering from carbon, nitrogen and oxygen. It permits the computation of matrix correction factors for the analysis of low energy X-ray emitters. In this paper, the first version of the simulation algorithm is presented as also the application to the analysis of reference organic material.