An evolutionary model of star formation in ULIRGs was created using existing star formation and radiative transfer codes (STARBURST99, RADMC and RADICAL) as building blocks. The results of the simulations are compared to data from two IRAS catalogs. From the simulations it is found that the dust surrounding the starburst region is made up from two components. There is a low optical depth (tau=0.1, which corresponds to 0.1% of the total dust mass), hot (T~400K) non-grey component close to the starburst (scale size 10pc) and a large scale, colder grey component (100pc, 75K) with a much larger column (tau=10). The simulations also show that starburst galaxies can be powered by massive star formation. The parameters for this star forming region are difficult to determine, since the IR continuum luminosity is only sensitive to the total UV input. Therefore, there is a degeneracy between the total starburst mass and the initial mass function (IMF) slope. A less massive star formation with a shallower IMF will produce the same amount of OB stars and therefore the same amount of irradiating UV flux. Assuming the stars are formed according to a Salpeter IMF (Psi(M) ~ M^-2.35), the star formation region should produce 10^9 Msun of stars (either in one instantaneous burst, or in a continuous process) in order to produce enough IR radiation.
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