Abstract: This invention relates to a process for the preparation of pimavanserin comprising: (i) providing an acid addition salt of pimavanserin; (ii) dissolving the acid addition salt of pimavanserin in an aqueous solvent to form an aqueous solution; (iii) washing the aqueous solution obtained in step (ii) with an organic solvent; and (iv) adding a base to the washed aqueous solution to form pimavanserin. The invention also relates to a process for the preparation of an acid addition salt of pimavanserin additionally comprising step (v) of converting pimavanserin into an acid addition salt of pimavanserin. The invention also relates to pimavanserin or an acid addition salt thereof obtainable by the process, and to the use of pimavanserin hydrochloride, pimavanserin hydrogen sulfate or pimavanserin acetate for preparing pimavanserin or an acid addition salt thereof.