The ground state configuration of the monochalcogenides of Sm and Eu is determined from total energy calculations using the self-interaction corrected local-spin-density approximation. The Sm chalcogenides, with the exception of SmO, are characterized by divalent f6 Sm ions, while all the Eu chalcogenides have divalent f7 Eu ions in the ground state. With pressure, the Eu and Sm chalcogenides exhibit isostructural transitions into an intermediate valent state, which in the total energy calculations is represented by localized f5 configurations on the Sm ions (f 6 on Eu ions) together with a partly occupied f-band at the Fermi level. The energy of the fundamental f → d transition, which determines the value of the semiconducting gap, is determined by total energy calculations of the charged rare earth ion (Eu+ or Sm+) in a supercell approach with one f-electron removed. The pressure coefficients are in excellent agreement with experiment, and the occurrence of isostructural transitions is intimately related to the closure of the band gap. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.