Ten phenolic acids, contained in the seeds of black cumin (Nigella sativa L.), flax (Linum usitatissimum L.), pomegranate (Punica granatum L.) and pumpkin (Cucurbita pepo L.) and their oil industry by-products, separated into the free, esterified, and insoluble-bound forms, were quantitatively analysed by reverse phase high performance liquid chromatography with photodiode array detector. The chromatographic data were interpreted using Principal Component Analysis (PCA). The PCA model with three principal components (PC1-PC2-PC3) fitted well with 12 examined plant samples, allowing their division into groups according to their origin. The total phenolic variables could be represented by two PCs and for the pattern recognition of the analysed samples, 13 phenolic variables are sufficient, including: free, esterified and insoluble-bound forms of gallic and syringic acids, free vanillic, insoluble bound p-coumaric, esterified p-hydroxybenzaldehide, and free and insoluble-bound forms of p-hydroxybenzoic and trans-synapic acids. This might have potential application in simplified screening of phenolic compounds in seeds and their oil industry by-products or in food component analysis or authenticity detection in such plant materials.