Origin of the low temperature excess heat capacity of isotopically substituted acetylsalicylic acid

► Cp of methyl-deuterated ASA crystals is measured between 2 and 80 K. ► Mono-methyldeuterated ASA (ASA-CH2D): Schottky peak in Cp below T  ≈  40 K. ► ΔE between the two lowest energy levels in the CH2D group is ≈3.15 meV. ► ΔCp between ASA-CD3, ASA-13CH3 and ASA-CH3 is minor in the examined T-range.