A positive ion composition measurement at the summer, high latitude mesopause in the presence of noctilucent clouds has revealed the existence of very massive positive ionospheric ions. The ions were assigned to be proton hydrates of which the heaviest ones contained up to 20 water molecules. These most massive ions were concentrated in a thin layer at an altitude of 90 km. A likely explantation of the existence of so heavy ionospheric ions is a strong local cooling within the layer in combination with a very low electron concentration. In spite of the high altitude of detection of these ions it is conceivable that they represented condensation nuclei eventually leading to the formation of visible notilucent clouds.