The decarbonization of high-temperature industrial combustion processes is possible using alternative fuels. Among them, hydrogen is considered as it can be produced from renewable sources. Compared to methane, hydrogen has a lower density and lower energy density for an equivalent volume. Its combustion is characterized by an increase of laminar burning velocity, flame stability and flame temperature, the latter in combustion with air but not necessarily with oxycombustion, and furthermore by higher water vapour content in flue gases. A progressive replacement of methane by hydrogen induces significant changes in flame structure and combustion features that need to be explored. The objective of this work is to study these effects on a coaxial turbulent diffusion flame by an experimental approach in a lab-scale facility, characterizing the consequences of hydrogen proportion increase in the (CH4 – H2) fuel blend up to pure hydrogen in oxycombustion. A complete flame stability diagram is established. The UV-VIS-near IR flame emission is recorded by spontaneous emission spectroscopy to determine the emitted species explaining flames appearance. Flames structures and lengths are obtained by OH* and CH* chemiluminescence imaging. The measurements of near wall temperature and total flux allow characterizing the heat transfer of the flames. Thermochemical kinetics calculations were also performed to help understanding the combustion performance of fuel blends and to analyze the experimental results. All results point out modifications of the oxyflames characteristics that must be known when transitioning gaseous fuel from natural gas to hydrogen.