We report on the spectroscopic identification and the long-awaited redshift
measurement of the heavily obscured, gravitationally lensed radio source P
KS 1830-211, which was first observed as a radio Einstein ring. The northea
st component of the doubly imaged core is identified, in our infrared spect
rum covering the wavelength range 1.5-2.5 mu m, as an impressively reddened
quasar at z = 2.507 +/- 0.002. The mass contained within the Einstein ring
radius is M(r<2.1 h(-1) kpc) = 6.3 x 10(10) h(-1) M-. for Omega(M)= 1 or M
(r< 2.4 h(-1) kpc) = 7.4 x 10(10) h(-1) M-. for Omega(M) = 0.3. Our redshif
t measurement, together with the recently measured time delay (Lovell et al
.), means that we are a step closer to determining H-0 from this lens. Conv
erting the time delay into H-0 by using existing models leads to high value
s of the Hubble parameter, H-0 = 65(-19)(+15) for Omega(M)= 1 and H-0= 76(-
10)(+18) for Omega(M) = 0.3. Since the lensing galaxy lies very close to th
e center of the lensed ring, improving the error bars on H-0 will require n
ot only a more precise time delay measurement but also very precise astrome
try of the whole system.