Context :

Aims : We report on the redshift of the lensing galaxy and of the quasar QJ 0158-4325 and on the lens model of the system .

Methods : A deep VLT/FORS2 spectrum and HST/NICMOS-F160W images are deconvolved .
From the images we derive the light profile of the lensing galaxy and an accurate relative astrometry for the system .
In addition we measure the flux ratio between the quasar images in the Mg II emission line to constrain the mass model .

Results : From the spectrum we measure the redshift of the lensing galaxy ( z = 0.317 \pm 0.001 ) and of the quasar ( z = 1.294 \pm 0.008 ) .
Using the flux ratio in the lens model allows to discard the SIE as a suitable approximation of the lens potential .
On the contrary the truncated-PIEMD gives a good fit to the lens and leads to a time delay of \Delta t _ { A - B } =-14.5 \pm 0.1 days , with H _ { 0 } =73 km s ^ { -1 } Mpc ^ { -1 } .

Conclusions : Using the flux ratio to constrain the mass model favors the truncated-PIEMD over the SIE , while ignoring this constraint leaves the choice open .