N-methyl-N-Ä(1S-)-1-phenyl-2-((3S)-3-hydroxypyrrolidin-1-yl)ethylÜ-2,2 -diphenylacetamide hydrochloride (I) is claimed with thermostable crystal modification. PREFERRED - Pref. method is: a soln. ofdiphenylacetylchloride is slowly added to soln. of 1-Ä(1S-3-hydroxypyroolidin-1-ylÜ-(2S)-2-methylamino-2- phenylethane whilst maintaining a constant low (pref. -5-10 degrees C) temp.. The raw product is recrystallised from a warm solvent to give (I) (m.pt. 220-225 degrees C).