  Exclusive electroproduction of rho^0 mesons has been measured using the ZEUS
detector at HERA in two Q^2 ranges, 0.25<Q^2<0.85 GeV^2 and 3<Q^2<30 GeV^2. The
low-Q^2 data span the range 20<W<90 GeV; the high-Q^2 data cover the 40<W<120
GeV interval. Both samples extend up to four-momentum transfers of |t|=0.6
GeV^2. The distribution in the azimuthal angle between the positron scattering
plane and the rho^0 production plane shows a small but significant violation of
s-channel helicity conservation, corresponding to the production of
longitudinally polarised (i.e. helicity zero) rho^0 mesons from transverse
photons. Measurements of the 15 combinations of spin-density matrix elements
which completely define the angular distributions are presented and discussed.
