Résumé : The helicity structure of the diffractive electroproduction of ρ mesons, e + p → e + ρ + Y, is studied in a previously unexplored region of large four-momentum transfer squared at the proton vertex, t: 0 < t′ < 3 GeV2, where t′ = t- tmin The data used are collected with the H1 detector at HERA in the kinematic domain 2,5 < Q2 < 60 GeV2, 40 < W < 120 GeV. No t dependence of the r0004 spin density matrix element is found. A significant t dependent helicity non-conservation from the virtual photon to the ρ meson is observed for the spin density matrix element combinations r005 + 2r115 and r001 + 2r111. These t dependences are consistently described by a perturbative QCD model based on the exchange of two gluons. © 2002 Elsevier Science B.V. All rights reserved.