We present a microscopic Monte Carlo description of the site-diluted b.c.c. frustrated Ising antiferromagnet Fe x Zn 1−x F 2 , with an emphasis on the random exchange Ising model ͑REIM͒ phase observed at high and intermediate magnetic concentrations, 0.3Շ x Ͻ 1.0, in a zero magnetic field. Thermodynamical, microscopic, and critical properties of the x = 0.48 system are found to be in good agreement with both zero-field experimental measurements and previous Monte Carlo results on the site-diluted s.c. nonfrustrated ferromagnetic Ising model. In particular, we contrast the results of the REIM behavior at x = 0.48 with those of the zero-field spin-glass phase observed at x = 0.25.