In this paper, the temporal evolution of auto-ignition (AI) of C2H4 and CH4 jets propagating into a NOx vitiated hot co-flow at high velocity and turbulence was studied. Simultaneous temporally-resolved planar laser-induced fluorescence (PLIF) experiments of OH and CH2O were carried out in a recently developed test rig for auto-ignition studies of turbulent non-premixed flows. Flame stabilization mechanisms were analyzed for both fuels at several operating conditions. The reaction progress of AI kernels as well as their apparent growth rate were evaluated. Results revealed that the stabilization mechanism (i.e. lifted flame or isolated kernel) strongly depends on the turbulent mixing, co-flow temperature and fuel composition. A further statistical analysis of the heat release rate (HRR) zones, calculated as the product of the CH2O- and OH-PLIF signals, delivered an indication on the different AI characteristics of C2H4 and CH4. An evaluation of the temporal evolution of the HRR for C2H4 jets provided a deeper insight about the reaction progress at different conditions of the co-flow, when Al was initiated by isolated kernels. Finally, estimations of the apparent growth rate of AI kernels indicated a faster propagation of C2H4 kernels when compared with those of CF4. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.