The technique of constructing optimal descriptors calculated with the Simplified molecular input line entry system (SMILES) is described. SMILES based optimal descriptors and descriptors calculated with molecular graphs (hydrogen filled graphs and graph of atomic orbitals) are compared in modeling done by means of quantitative structure - property/activity relationships (QSPR/QSAR). QSPR/QSAR models for normal boiling points of organic compounds, mutagenicity of heteroaromatic amines, toxicity, and anti-HIV-1 potentials of TIBO and HEPT derivatives are described in details. Possible ways to improve the SMILES based concept of QSPR/QSAR analyses are discussed.