We observed a decrease of the number of effective emission centers Mn2+ in the aged ZnS:Mn electroluminescence (EL) devices compared to the fresh EL devices using the electron-paramagnetic-resonance technique. Such phenomena can take place during the operation of the EL device, since the isolated Mn can easily diffuse into another site and forming cluster. Another possible explanation is that Mn2+ changes into Mn1+ or Mn3+ by transferring the electronic charge of the isolated Mn2+ to the neighboring Mn ions via sulfur and/or sulfur vacancy. As a result, luminance is lowered due to the decrease in the number of efficient emission centers of isolated Mn2+.