GRAVITATIONAL lensing can be used to derive fundamental cosmological parameters, provided that the redshift of the lens and the structure of its potential well are known(1). The radio source PKS1830-211-a lensed quasar(2-5) whose components are separated by about 1 arcsecond-is ideal for such an examination, but until recently the redshift of the lens has been unknown. Here we report the detection and identification of 12 molecular absorption lines in the spectrum of PKS1830-211; the lines originate in the lensing source, which is probably a spiral galaxy, at a redshift of similar to 0.89, Only one of the lensed components is covered by the intervening molecular gas; when combined with the strong variability in the quasar, this creates a unique opportunity to measure the difference in the light travel time for the two components, which depends in part on the Hubble constant and the cosmic deceleration parameter.