Context :

Aims : The interstellar medium ( ISM ) has a multiphase structure characterized by gas , dust and molecules .
The gas can be found in different charge states : neutral , low-ionized ( warm ) and high-ionized ( hot ) .
It is possible to probe the multiphase ISM through the observation of its absorption lines and edges in the X-ray spectra of background sources .

Methods : We present a high-quality RGS spectrum of the low-mass X-ray binary GS 1826 - 238 with an unprecedent detailed treatment of the absorption features due to the dust and both the neutral and ionized gas of the ISM .
We constrain the column density ratios within the different phases of the ISM and measure the abundances of elements such as O , Ne , Fe and Mg .

Results : We found significant deviations from the proto-Solar abundances : oxygen is over-abundant by a factor 1.23 \pm 0.05 , neon 1.75 \pm 0.11 , iron 1.37 \pm 0.17 and magnesium 2.45 \pm 0.35 .
The abundances are consistent with the measured metallicity gradient in our Galaxy : the ISM appears to be metal-rich in the inner regions .
The spectrum also shows the presence of warm/hot ionized gas .
The gas column has a total ionization degree less than 10 % .
We also show that dust plays an important role as expected from the position of GS 1826 - 238 : most iron appears to be bound in dust grains , while 10 - 40 % of oxygen consists of a mixture of dust and molecules .

Conclusions :