Complementary methods based on three mass spectrometric ionization techniques are presented for the determination of the thermal decomposition products of the α‐dipeptide sweetener aspartame. Thermal decomposition products generated in acidic aqueous model systems were separated by ion‐pair high‐performance liquid chromatography and either (1) isolated and concentrated on solid‐phase extraction columns for determination by direct insertion probe/electron impact mass spectrometry; (2) determined by gas chromatography/mass spectrometry asN‐trifluoroethyl‐O‐trimethylsilyl polyamino alcohol derivatives; or (3) determined directly via Thermospray liquid chromatography/mass spectrometry. These procedures resulted in the elucidation and structural determination of four compounds: (1) β‐aspartame, a non‐sweet constitutional isomer of aspartame; (2) its anhydro analogue, methyl‐3‐phenyl‐2‐(3‐amino‐2,5‐dioxo‐1‐pyrrolidinyl propanoate; (3) the non‐sweet β‐L‐aspartyl‐L‐phenylalanine; and (4) its anhydro analogue, 3‐phenyl‐2‐(3‐amino‐2,5‐dioxo‐1‐pyrrolidinyl)propanoic acid.