  We have measured several 2p $\rightarrow$ 1s transition energies in
core-excited boron-like ions of sulfur and argon. The measurements are
reference-free, with an accuracy of a few parts per million. The x-rays were
produced by the plasma of a an electron-cyclotron resonance ion source and were
measured by a double-crystal x-ray spectrometer. The precision obtained for the
measured 1s2s22p2 J -- 1s2 2s2 2p J ' lines is $\approx$4 ppm for sulfur and
$\approx$2 ppm for argon. The line energies are compared to relativistic atomic
structure calculations performed with the mdfgme multi-configuration Dirac-Fock
code. This comparison is used for line identification and test the theoretical
methods, which reach an agreement with experimental data up to 15 meV. The
theoretical calculations have been extended to C+, Si9+, Cr19+ and Fe21+, which
were the only B-like ions where such transitions were measured up to now.
