The relaxation of Yb3+ in YBa2Cu3Ox (6<x<7) was studied using electron paramagnetic resonance. It was
found that both electronic and phononic processes contribute to the Yb3+ relaxation. The phononic part of the
relaxation has an exponential temperature dependence, which can be explained by a Raman process via the coupling to high-energy (~500 K) optical phonons or an Orbach-type process via the excited vibronic levels of the Cu2+ ions (localized Slonczewski-modes). In a sample with a maximum oxygen doping x=6.98, the electronic part of the relaxation follows a Korringa law in the normal state and strongly decreases below Tc. Comparison of the samples with and without Zn doping proved that the superconducting gap opening is responsible for the sharp decrease of Yb3+ relaxation in YBa2Cu3O6.98. It was shown that the electronic part of the Yb3+ relaxation in the superconducting state follows the same temperature dependence as 63Cu and 17O nuclear relaxations despite the huge difference between the corresponding electronic and nuclear relaxation rates.