A comparison between angle resolved photoemission data and ab initio pseudopotential calculations for a structural model of the As covered Si(100) surface is presented. After a coverage of one monolayer the Si(100):As surface shows a 2×1 low energy electron diffraction pattern. The arsenic acts as an effective passivating layer and the adsorption of further arsenic is highly reduced. We propose a symmetric As–As dimer model for the Si(100):As 2×1 surface derived from energy minimization calculations. The major feature of the electronic structure of this model is the occurrence of occupied π and π* surface state bands derived from bonding and antibonding combinations of the dangling hybrids on the As atoms. The calculated dispersions for the π and π* bands are found to be in very good agreement with the experimental surface state dispersions. The results discussed provide an example of how angle resolved photoemission in combination with ab initio pseudopotential calculations can lead to a conclusive determination of the electronic and atomic structure of surfaces.