The large uncertainties associated with measuring the amount of high temperature emission in solar active regions represents a significant impediment to making progress on the coronal heating problem .
Most current observations at temperatures of 3 MK and above are taken with broad band soft X-ray instruments .
Such measurements have proven difficult to interpret unambiguously .
Here we present the first spectroscopic observations of the Fe xviii 974.86 Å emission line in an on-disk active region taken with then SUMER instrument on SOHO .
Fe xviii has a peak formation temperature of 7.1 MK and provides important constraints on the amount of impulsive heating in the corona .
Detailed evaluation of the spectra and comparison of the SUMER data with soft X-ray images from the XRT on Hinode confirm that this line is unblended .
We also compare the spectroscopic data with observations from the AIA 94 Å channel on SDO .
The AIA 94 Å channel also contains Fe xviii , but is blended with emission formed at lower temperatures .
We find that is possible to remove the contaminating blends and form relatively pure Fe xviii images that are consistent with the spectroscopic observations from SUMER .
The observed spectra also contain the Ca xiv 943.63 Å line that , although a factor 2 to 6 weaker than the Fe xviii 974.86 Å line , allows us to probe the plasma around 3.5 MK .
The observed ratio between the two lines indicates ( isothermal approximation ) that most of the plasma in the brighter Fe xviii active region loops is at temperatures between 3.5 and 4 MK .