In this paper we examine the properties of arsenic in silicon, using ab initio calculations and a statistical theory. Good agreement is found between theory and experiment for the electronic concentration as a function of temperature and total arsenic concentration. We show that for low arsenic concentrations, full activation is the equilibrium condition. In equilibrium, the neutral complex composed of a lattice vacancy surrounded by four arsenic (VAs4) is the dominant means by which high concentrations of arsenic are rendered inactive. Under constrained equilibrium conditions in which VAs4 cluster formation is prohibited, we show that VAs3Si1 cluster populations increase dramatically and can account for nearly the same degree of compensation as the VAs4 clusters. Even VAs2 clusters alone can account for substantial deactivation in the absence of VAs3 and VAs4 clusters. These smaller complexes are essential not only to the establishment of equilibrium, since SiAs4 clusters are extremely rare, but can also explain some degree of deactivation, even if the formation of VAs4 clusters are kinetically inhibited.