The present invention relates to a process for the preparation of the compound N-(3,5-dimethylphenyl)-N'-(2-trifluoromethylphenyl) guanidine of formula (I) which comprises reacting a salt of a compound of formula (II) or a mixture of the salt of the compound of formula (II) and a compound of formula (II) with a compound of formula (III), in the presence of a polar organic solvent； a crystalline solid form of the compound of formula (I), in particular crystalline solid form A； to a pharmaceutical composition comprising them； and to their use as a medicament, in particular to the treatment of a condition mediated by Rho-GTPase cell proteins.
