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The very inner structure of massive YSOs is difficult to trace.
With conventional observational methods we often identify structures still
several hundreds of AU in size. But we also need information about the
innermost regions where the actual mass transfer onto the forming high-mass
star occurs. An innovative way to probe these scales is to utilise
mid-infrared interferometry. Here, we present results of our MIDI GTO
programme at the VLTI. We observed 10 well-known massive YSOs down to scales
of 20 mas. We clearly resolve these objects which results in low visibilities
and sizes in the order of 30 - 50 mas. Thus, with MIDI we can for the first
time quantify the extent of the thermal emission from the warm circumstellar
dust and thus calibrate existing concepts regarding the compactness of such
emission in the pre-UCHII region phase. We will discuss the indications for
circumstellar disks and for bloated central objects indicating the accretion
history of those massive YSOs. We demonstrate how these interferometric
results can help to resolve the ambiguities normally arising from simple SED
fitting. Finally, an outlook onto the capabilities of the planned 2nd
generation VLTI instruments in the massive YSO context will be given.
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