We observed the decomposition of the hydrochloride salt of α-pyrrolidinoheptanophenone (α-PHPP-HCl), a newly distributed pyrrolidine-type cathinone derivative when 2.5 ng of this substance was placed in glass test tubes and stored in a refrigerator for 3 days. To further investigate this phenomenon, we studied the (i) time course of the residual ratios of α-PHPP-HCl when a small amount (10 μg) of α-PHPP-HCl was stored in glass vials in air at room temperature; (ii) identification of the decomposition products of α-PHPP-HCl; (iii) effect of air on the decomposition process; (iv) effect of the added amounts of α-PHPP-HCl on its decomposition; and (v) comparison of the stability between various cathinone derivatives and their decomposition products. The decomposition of α-PHPP-HCl occurred in air and increased with time. Two possible decomposition products, α-(2″-oxopyrrolidino)heptanophenone and α-PHPP-N-oxide, were identified. These products were formed by oxygen in air because the yield significantly decreased by storing them in a vacuum desiccator. With the decrease in the amount of α-PHPP-HCl, the residual ratios decreased and amount of the decomposition products increased. This indicates that the decomposition of α-PHPP-HCl occurred on the upper surface of the samples. The hydrochloride salts of other cathinone derivatives were also unstable in air, and the residual ratios observed were different depending on the compounds. The pyrrolidine-type cathinone derivatives afforded two types of decomposition products, which were presumed to be 2″-oxo and N-oxide derivatives, similar to α-PHPP-HCl. In contrast, secondary amine-type cathinone derivatives showed different decomposition patterns, possibly including the dealkylated derivative. These findings may be very useful for the future toxicological analysis of cathinone derivatives.