Abstract The modified Ruland method  has been applied to two glassy systems having the following chemical compositions (mol%):SiO 266.75, Li 2O23.45, ZnO 8.00, K 2O 1.00, P 2O 5 0.80; and SiO 2 70, Li 2O 17, TiO 2 10, Al 2O 3 3, in order to determine the crystallinity of partially crystallized materials obtained from the initial glasses. For the first system it has been shown that the two main approximations present in the relations of the method are well justified and this result has a general validity. For the second system a comparison has been carried out with the crystallinity data obtained by simply measuring the XRD peak heights “in situ”, at high temperature. The results are ingood agreement. We have shown that, when a tolerably low overlapping among the XRD peaks of different crystalline phases is present, multi-phase transformation can be studied.