The structure of neoclassical tearing modes in ASDEX Upgrade is analysed bythe temperature profiles of the associated magnetic islands. The electrontemperature profiles are measured by electron cyclotron emission spectroscopyand are evaluated based on modelled temperature profiles. They are obtainedfrom the numerically solved heat flux equation using realistically modelledhelical magnetic fluxes and taking into account the fact that, incollisionless plasmas, the parallel heat transport is limited by the electronthermal velocity. The observed magnetic islands have a width of 6-8 cm andare asymmetric in accordance with our model for the perturbed flux. Thenonlinear stability parameter Δ'(W) tends to have negative values. Theobserved temperature profiles exhibit substructures inside the magneticislands instead of the theoretically expected complete flattening in theO-point region.