The binary systems of urea with polyethylene glycols 6000 and 4000 show inclusion compounds with higher melting points than
the two components (m.p. 143 and 142.5°C resp.). From the melt unstable forms crystallize beside the stable crystal modifications. These have also
been identified by FTIR microscopy and X-ray powder diffractometry. The phase diagrams are uncommon in so far as the inclusion
compounds do not form eutectics but monotectics with both components. The inclusion compounds of the two polyethylene glycols
with urea are isomorphous and form a series of mixed crystals following the Roozeboom I type of diagram.