A comparison of the flame structure for two different fuels, dodecane and oxymethylene dimethyl ether (OMEX), has been performed under condition of Spray A of the Engine Combustion Network (ECN). The experiments were carried out in a constant pressure vessel with wide optical access, at high pressure and temperature and controlled oxygen concentration. The flame structure analysis has been performed by measuring the formaldehyde and OH radical distributions using planar Laser-Induced Fluorescence (PLIF) techniques. To complement the analysis, this information was combined with that obtained with highspeed imaging of OH * chemiluminescence radiation in the UV. Formaldehyde molecules are excited with the 355-nm radiation from the third harmonic of a Nd:YAG laser, whilst OH is excited with a wavelength of 281.00-nm from a dye laser. In both cases, the beam was transformed into a laser sheet in order to excite an axial flame plane and the fluorescence radiation was collected with an intensified camera (ICCD) and proper filtering. Consequently, two-dimensional maps in the axial flame plane were obtained at different instants after the start of injection (ASOI). Signal from both formaldehyde and OH chemical species can be compared, in order to analyze spatial distribution and interaction. When dodecane and OMEX are compared, several differences arise. The second one presents larger lift-off length but remarkably shorter flame length. Additionally, it has been possible to appreciate for this fuel a lower amount of soot formation during combustion.