Using an X-pinch configuration, we have determined that micropinches produced by exploding-wire z pinches can have densities approaching solid density and temperatures of 0.5-1.8 keV, depending upon the wire material used. These plasma parameters, determined from x-ray spectra recorded using an x-ray streak camera, vary drastically on time scales ranging from <10 to 100 ps. Computer simulations require radiation loss to reproduce the observed plasma implosion, suggesting that a radiative-collapse hypothesis for micropinch plasma formation may be correct.