We present UV to soft X-ray spectra of 31 radio-quiet quasars , comprising data from the IUE ULDA database and the ROSAT pointed observation phase .
90 % of the sample members show a soft X-ray excess above an underlying hard X-ray power law spectrum .
Particularly for the steep X-ray spectrum ( \alpha _ { energy } > 1.5 ) , low redshift subsample ( 17 objects ) the X-ray spectral power law index is strongly correlated with the optical to X-ray broad-band spectral index \alpha _ { ox } , indicating that the main contribution to the soft X-ray and optical emission is due to the same emission component .
We model the UV/soft X-ray spectra in terms of thermal emission from a geometrically thin \alpha -accretion disk .
The structure and radiation field of the disk is calculated self-consistently and Compton scattering is treated by the Kompaneets equation .
All relativistic effects on the disk structure and the emergent disk spectrum are included .
Satisfactory spectral fits of the UV and soft X-ray continuum spectra are achieved when additional non-thermal hard X-ray and IR power law emission components are taken into account .
The UV and soft X-ray spectra are well described by emission resulting from accretion rates in the range \sim 0.1 to \sim 0.3 times the Eddington accretion rate .
Low mass/low redshift objects are found to accret at < 0.15 \dot { M } _ { Eddington } .
Correlations of the accretion disk parameters with \alpha _ { ox } are discussed .