A rather simple process for preparing BaFCl:Eu phosphor of contral-lable particle size has beend developed.Refering to the phase diagram of BaCl

2

-BaF

2

-H

2

O system

the matrix compound BaFCl was synthesized by "homogeneous" precipitation. BaFCl thus obtained is tiny crystallite with a mean size smaller than 0.5μm EuCl

3

and KC1 were added to the BaFCl and heated in air to form the Eu

2+

doped phosphor.The reaction mechanism of thermal reduction decomposition of EuCl

3

in pure state and in the system of BaFCl-EuCl

3

has been studied by equilibrium vapour pressure measurement and solid phase composition and structure determination.X-ray intensifying screens for diagnostic radiography made with this phosphor exhibit a purple-blue emission spectrum being enveloped by the sensitivity curve of the conventional blue sensitive film and have a high speed and good image quality.