The present invention relates to crystalline and amorphous forms of 6-fluoro-2-[4-(pyridin-2-yl)but-3-yn-1-yl]imidazo[1,2-a]pyridine mono-phosphate salt and methods of making these solid forms thereof. The invention compounds are modulators of mGlu5 which are useful for the treatment or prevention of central nervous system disorders as well as other disorders modulated by mGlu5 receptors. The invention is also directed to pharmaceutical compositions and the use of such compounds in the manufacture of medicaments, as well as to the use of such compounds for the prevention and treatment of such diseases in which mGlu5 is involved.