We present X-ray observations of two high redshift clusters of galaxies carried out with the Beppo SAX satellite .
One cluster , RXJ 0152.7 - 1357 at z \simeq 0.83 , was selected from the ROSAT Deep Cluster Survey sample , as one of the most X-ray luminous systems known at z > 0.5 .
The optical and ROSAT-PSPC data show a complex morphology with at least two cores .
Our SAX observations yield a gas temperature kT = 6.46 ^ { +1.74 } _ { -1.19 } keV and a metallicity A = 0.53 ^ { +0.29 } _ { -0.24 } , with a prominent iron K _ { \alpha } line .
The second cluster , MS 2053.7 - 0449 at z \simeq 0.58 , was selected from the EMSS sample .
Given the poor statistics no constraints on the metallicity can be derived from the present data .
Large uncertainties are associated to the gas temperature ( kT = 6.7 ^ { +6.8 } _ { -2.3 } keV ) , which has been obtained after fixing the abundance to 0.3 solar .
Combining these results with those obtained for similarly high redshift ( z \geq 0.5 ) clusters of galaxies with broad band X-ray spectra , we discuss the high redshift L _ { bol } - T relationship .
The data can easily be accommodated with a non-evolving ( or mildly evolving ) L _ { bol } - T relation , a result which , when combined with the little observed evolution in the bulk of the X-ray cluster population , gives support to low \Omega cosmological models .