A brief survey is given of the results of extended investigations into certain systems of anomalous mixed crystals between uranium compounds and fluorides of the alkalis and alkaline earths. The systems in question are as follows: NaF–Na 2 U 2 O 7 , LiF–Li 2 UO 4 , KF–K 2 UO 4 , NaF–UO 2 (OH) 2 , CaF 2 –CaUO 4 , CaF 2 –UO 2 . These mixed crystals were subjected to examination under the polarizing microscope, by x-rays and by spectral photometry. In numerous cases it was possible to deduce the mechanism of intergrowth and this can serve as a base on which a later discussion of the absorption and luminescence phenomena of these systems can be founded.