In this paper, two cathinone derivatives, 4F-3Me-α-PVP and N-ethylheptedrone, seized on the illegal drug market in Poland, were described and characterized by various instrumental analytical methods. The compounds were characterized by electrospray ionization mass spectrometry, high-resolution mass spectrometry, gas chromatography–mass spectrometry, infrared spectroscopy, X-ray crystallography, thermogravimetric analysis, differential scanning calorimetry and nuclear magnetic resonance spectroscopy. The two tested compounds were confirmed as 1-(4-fluoro-3-methylphenyl)-2-(pyrrolidin-1-yl)pentan-1-one and N-ethyl-2-amino-1-phenylheptan-1-one hydrochlorides; both are cathinone derivatives available on the market for new psychoactive substances (NPS). The obtained analytical data should be useful for forensic and toxicological purposes in the rapid and reliable identification of compounds.