A simple theory of exciton tunneling in GaAs1-xPx:N is presented. Excitons bound to nitrogen (Nx) tunnel to sites of lower energy within a disorder-broadened line. They continue to tunnel until there are no sites of lower energy within an effective tunneling radius. This radius grows slowly with time. The theory explains the absence of luminescence from NN pairs at low temperatures and the nonthermal Nx luminescence line shape. Monte Carlo simulations confirm the dynamics of the tunneling. © 1984 The American Physical Society.