Local structural arrangements and corresponding dynamical effects in relaxor ferroelectrics PbSc1/2Nb1/2O3 and PbSc1/2Ta1/2O3 were studied by Raman spectroscopy. Polarized Raman spectra of single crystals were measured at different temperatures in several scattering configurations. The peaks observed are assigned on the basis of normal-mode calculations. The effect of small structural deviations on the atomic vibrations is analysed by calculating the modes of structural units at various atomic positions. It is shown that doubling the structural unit is always possible on a local scale of a few unit cells. The presence of two Raman peaks originating from the F2u mode shows that electron-phonon coupling occurs in PbSc1/2B''1/2O3, thus leading to dynamical off-centre-symmetrical structural fluctuations. The structural perturbation consists of the non-coplanarity of Pb and O atoms in the planes perpendicular to the body diagonal and deviations of B cations from the centre of BO6 octahedra. The intensity ratio of the two peaks is sensitive to the degree of orientational ordering of the lone pairs of Pb atoms within the Pb-O planes, which is related to the degree of compositional B'/B'' ordering. The temperature dependence of the peaks arising from asymmetrical O-B-O bending, BO3 translation and Pb-localized modes shows that on cooling different structural changes are preferential in the two materials studied. In PbSc1/2Nb1/2O3 the lowering of the temperature favours the off-centre deviations of the B cations. In PbSc1/2Ta1/2O3 the temperature decrease stabilizes the non-coplanarity of the Pb and O atoms in the planes perpendicular to the body diagonal and enhances the correlation between the electronic lone pairs of Pb.