The present invention relates to crystalline forms of (3S)-3-[N—(N′-(2-tert-butylphenyl)oxamyl)alaninyl]amino-5-(2′,3′,5′,6′-tetrafluorophenoxy)-4-oxopentanoic acid (see formula I). The invention further relates to pharmaceutical compositions comprising such crystalline forms and to the use of said pharmaceutical compositions and said crystalline forms in the treatment of various conditions, particularly in the treatment of liver fibrosis.