Temperature-dependent positron lifetime measurements in ceramic Y1−xPrxBa2Cu3O7−δ samples reveal positron trapping, in particular at low temperature and for small x. Positrons appear to be completely delocalized for T∼400 K and higher. At high temperatures the lifetime for YBa2Cu3O7−δ and PrBa2Cu3O7−δ is identical (∼165 ps) and close to the theoretical value. For these reasons a two-dimensional angular correlation of annihilation radiation (2D-ACAR) spectrum was measured in YBa2Cu3O7 at T=400 K. The spectrum width confirms the delocalization of the positron and the 2D-ACAR shows, apart from the one-dimensional Fermi surface due to CuO chains, a smaller Fermi surface sheet centered around the S point, in the first Brillouin zone.