We present the first detailed X-ray observations of optically-selected clusters at high redshift .
Two clusters , Cl 1324+3011 at z = 0.76 
and Cl 1604+4304 at z = 0.90 , were observed with XMM-Newton .
The optical center of each cluster is coincident with an extended X-ray source whose emission is detected out to a radius of \sim 0.5 ~ { } h _ { 70 } ^ { -1 } Mpc .
The emission from each cluster appears reasonably circular , with some indication of asymmetries and more complex morphologies .
Similarly to other optically-selected clusters at redshifts of z \gtrsim 0.4 , both clusters are modest X-ray emitters with bolometric luminosities of only L _ { x } ^ { bol } = 1.4 - 2.0 \times 10 ^ { 44 } ~ { } h _ { 70 } ^ { -2 } erg s ^ { -1 } .
We measure gas temperatures of T = 2.88 ^ { +0.71 } _ { -0.49 } keV for Cl 1324+3011 and 2.51 ^ { +1.05 } _ { -0.69 } keV for Cl 1604+4304 .
The X-ray properties of both clusters are consistent with the high-redshift L _ { x } - T relation measured from X-ray–selected samples at z \geq 0.5 .
However , based on the local relations , their X-ray luminosities and temperatures are low for their measured velocity dispersions ( \sigma ) .
The clusters are cooler by a factor of 2–9 compared to the local \sigma - T 
relation .
We briefly discuss the possible explanations for these results .